Add a missing `@since 3.9.0` to the PHPDoc for `wp_register_plugin_realpath()`.

See #16953, #27700.

Built from https://develop.svn.wordpress.org/trunk@28011


git-svn-id: http://core.svn.wordpress.org/trunk@27841 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE: show an error in the console when a plugin attempts to use the 'wpdialogs' plugin but wpdialog.js is not enqueued, fixes #16284
Built from https://develop.svn.wordpress.org/trunk@28012


git-svn-id: http://core.svn.wordpress.org/trunk@27842 1a063a9b-81f0-0310-95a4-ce76da25c4cd