Use _callback, rather than an unhelpful 2, for a preg_replace callback...

Use _callback, rather than an unhelpful 2, for a preg_replace callback function name. Add proper phpdoc. see #20369.

git-svn-id: https://core.svn.wordpress.org/trunk@20382 1a063a9b-81f0-0310-95a4-ce76da25c4cd