Document the new JSON error constants in PHP 5.5, since the naughty person who
implemented them didn't add them to UPGRADING. :)

Fixes doc bug #65803 (JSON errors missing a value).


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@331633 c90b9560-bf6c-de11-be94-00142212c4b1