Update constants documentation for 5.6.


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Some updates from UPGRADING. Still working on them.


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@333676 c90b9560-bf6c-de11-be94-00142212c4b1
PHP 5.6: json_decode() changes.

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@333677 c90b9560-bf6c-de11-be94-00142212c4b1