Add precision on documentation.

README.md

@@ -5228,7 +5228,8 @@ Read a word from the file 'f' and set 'v' with this word.
 A word is separated from another by the list of characters in the array 'separator'.
 (Example: "^ \t.\n").
 It is highly recommended for separator to be a constant string.
-'separator' shall be at most composed of 100 characters (as bytes).
+'separator' shall be at most composed of 100 bytes.
+It works with UTF8 stream with the restriction that the separator character shall only be ASCII character.
 
 ##### void string\_fputs(FILE \*f, const string\_t v)