[ticket/11711] Improve coding and comments of character check

PHPBB3-11711

phpBB/includes/message_parser.php

@@ -1195,7 +1195,7 @@ class parse_message extends bbcode_firstpass
 		}
 
 		// Check for out-of-bounds characters that are currently
-		// not supported by utf8_bin
+		// not supported by utf8_bin in MySQL
 		if (preg_match_all('/[\x{10000}-\x{10FFFF}]/u', $this->message, $matches))
 		{
 			$character_list = implode('<br />', $matches[0]);