Add note about messages going to stderr and the implication for pip…

…ing (#1768)

* Add note about messages going to STDERR and the implication for piping

* Minor wording change

* Wording and capitalization changes

docs/formats.md

@@ -117,6 +117,11 @@ These formats are mainly intended for piping or exporting your journal to other
 programs. Even so, they can still be used in the same way as any other format (like
 written to a file, or displayed in your terminal, if you want).
 
+!!! note
+You may see boxed messages like "2 entries found" when using these formats, but
+those messages are written to `stderr` instead of `stdout`, and won't be piped when
+using the `|` operator.
+
 ### JSON
 
 ``` sh