Inline documentation improvements for `wp_get_playlist()`.

* Adds a missing `@param` description in the PHPDoc.
* Adds hook documentation for the `post_playlist` filter.
* Other comment cleanup.

Built from https://develop.svn.wordpress.org/trunk@27240


git-svn-id: http://core.svn.wordpress.org/trunk@27097 1a063a9b-81f0-0310-95a4-ce76da25c4cd