Update commentary of STRING_CHAR and FETCH_MULTIBYTE_CHAR

* src/character.h (STRING_CHAR):
* src/buffer.h (FETCH_MULTIBYTE_CHAR): Update commentary: these
two macros no longer do any character unification, so the caveats
in those comments are no longer pertinent.

src/buffer.h

@@ -1182,17 +1182,7 @@ buffer_has_overlays (void)
 
 /* Return character code of multi-byte form at byte position POS.  If POS
    doesn't point the head of valid multi-byte form, only the byte at
-   POS is returned.  No range checking.
-
-   WARNING: The character returned by this macro could be "unified"
-   inside STRING_CHAR, if the original character in the buffer belongs
-   to one of the Private Use Areas (PUAs) of codepoints that Emacs
-   uses to support non-unified CJK characters.  If that happens,
-   CHAR_BYTES will return a value that is different from the length of
-   the original multibyte sequence stored in the buffer.  Therefore,
-   do _not_ use FETCH_MULTIBYTE_CHAR if you need to advance through
-   the buffer to the next character after fetching this one.  Instead,
-   use either FETCH_CHAR_ADVANCE or STRING_CHAR_AND_LENGTH.  */
+   POS is returned.  No range checking.  */
 
 INLINE int
 FETCH_MULTIBYTE_CHAR (ptrdiff_t pos)

src/character.h

@@ -308,10 +308,7 @@ enum
       }									\
   } while (false)
 
-/* Return the character code of character whose multibyte form is at
-   P.  Note that this macro unifies CJK characters whose codepoints
-   are in the Private Use Areas (PUAs), so it might return a different
-   codepoint from the one actually stored at P.  */
+/* Return the character code of character whose multibyte form is at P.  */
 
 #define STRING_CHAR(p)						\
   (!((p)[0] & 0x80)						\