Add opcache documentation.


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@330572 c90b9560-bf6c-de11-be94-00142212c4b1
Link directly to the ::class documentation.

I would have done this when I moved ::class up to a sect2, but I already had my
opcache changes pending. If only we used Git and I could have done a partial
commit...


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@330573 c90b9560-bf6c-de11-be94-00142212c4b1