Clarify json_last_error_msg()'s return values.

Fixes doc bug #67548 (Docs says json_last_error_msg should return null, but it
returns 'No Error').


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@334064 c90b9560-bf6c-de11-be94-00142212c4b1