docs/libcurl/opts: clarify the return values

Expand a little.

- mention the type name of the return code
- avoid stating which exact return codes that might be returned, as that
  varies over time, builds and conditions
- avoid stating some always return OK
- refer to the manpage documenting all the return codes

Closes curl#15900

docs/libcurl/opts/CURLINFO_ACTIVESOCKET.md

@@ -78,4 +78,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_APPCONNECT_TIME.md

@@ -67,4 +67,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_APPCONNECT_TIME_T.md

@@ -68,4 +68,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CAINFO.md

@@ -64,4 +64,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CAPATH.md

@@ -67,4 +67,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CERTINFO.md

@@ -103,4 +103,7 @@ Transport support added in 7.79.0. mbedTLS support added in 8.9.0.
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CONDITION_UNMET.md

@@ -76,4 +76,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CONNECT_TIME.md

@@ -63,4 +63,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CONNECT_TIME_T.md

@@ -66,4 +66,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CONN_ID.md

@@ -66,4 +66,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CONTENT_LENGTH_DOWNLOAD.md

@@ -70,4 +70,7 @@ Deprecated since 7.55.0.
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CONTENT_LENGTH_DOWNLOAD_T.md

@@ -63,4 +63,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CONTENT_LENGTH_UPLOAD.md

@@ -69,4 +69,7 @@ Deprecated since 7.55.0.
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CONTENT_LENGTH_UPLOAD_T.md

@@ -62,4 +62,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_CONTENT_TYPE.md

@@ -71,4 +71,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_COOKIELIST.md

@@ -78,4 +78,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_EARLYDATA_SENT_T.md

@@ -72,4 +72,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_EFFECTIVE_METHOD.md

@@ -68,4 +68,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_EFFECTIVE_URL.md

@@ -64,4 +64,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_FILETIME.md

@@ -73,4 +73,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_FILETIME_T.md

@@ -74,4 +74,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_FTP_ENTRY_PATH.md

@@ -70,4 +70,7 @@ Works for SFTP since 7.21.4
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_HEADER_SIZE.md

@@ -61,4 +61,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_HTTPAUTH_AVAIL.md

@@ -71,4 +71,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_HTTPAUTH_USED.md

@@ -73,4 +73,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_HTTP_CONNECTCODE.md

@@ -61,4 +61,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_HTTP_VERSION.md

@@ -57,4 +57,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_LASTSOCKET.md

@@ -78,4 +78,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_LOCAL_IP.md

@@ -69,4 +69,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_LOCAL_PORT.md

@@ -68,4 +68,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_NAMELOOKUP_TIME.md

@@ -64,4 +64,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_NAMELOOKUP_TIME_T.md

@@ -65,4 +65,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_NUM_CONNECTS.md

@@ -61,4 +61,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_OS_ERRNO.md

@@ -66,4 +66,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_POSTTRANSFER_TIME_T.md

@@ -67,4 +67,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_PRETRANSFER_TIME.md

@@ -69,4 +69,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_PRETRANSFER_TIME_T.md

@@ -71,4 +71,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_PRIMARY_IP.md

@@ -69,4 +69,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_PRIMARY_PORT.md

@@ -63,4 +63,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_PRIVATE.md

@@ -64,4 +64,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_PROTOCOL.md

@@ -73,4 +73,7 @@ Deprecated since 7.85.0.
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_PROXYAUTH_AVAIL.md

@@ -72,4 +72,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_PROXYAUTH_USED.md

@@ -76,4 +76,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).

docs/libcurl/opts/CURLINFO_PROXY_ERROR.md

@@ -101,4 +101,7 @@ int main(void)
 
 # RETURN VALUE
 
-Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+curl_easy_getinfo(3) returns a CURLcode indicating success or error.
+
+CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
+libcurl-errors(3).