Add comment explaining __mb_sb_limit trick here.

lib/libc/locale/utf8.c

@@ -65,6 +65,11 @@ _UTF8_init(_RuneLocale *rl)
 	__wcsnrtombs = _UTF8_wcsnrtombs;
 	_CurrentRuneLocale = rl;
 	__mb_cur_max = 6;
+	/*
+	 * UCS-4 encoding used as the internal representation, so
+	 * slots 0x0080-0x00FF are occuped and must be excluded
+	 * from the single byte ctype by setting the limit.
+	 */
 	__mb_sb_limit = 128;
 
 	return (0);