doc(string): add doc for String::char_at

Author: Yu-zh

string/string.mbt

@@ -391,12 +391,14 @@ pub fn String::offset_of_nth_char(
 }
 
 ///|
-/// Returns the Unicode character at the given index.
-pub fn String::char_at(self : String, index : Int) -> Char {
-  guard index >= 0 && index < self.length() else {
-    abort("index out of bounds")
+/// Returns the Unicode character at the given offset.
+/// 
+/// This has O(1) complexity.
+pub fn String::char_at(self : String, offset : Int) -> Char {
+  guard offset >= 0 && offset < self.length() else {
+    abort("offset out of bounds")
   }
-  self.unsafe_char_at(index)
+  self.unsafe_char_at(offset)
 }
 
 ///|