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.


git-svn-id: https://develop.svn.wordpress.org/trunk@27240 602fd350-edb4-49c9-b593-d223f7449a82
Spacing fixes for code introduced in [27239].

See #26631


git-svn-id: https://develop.svn.wordpress.org/trunk@27241 602fd350-edb4-49c9-b593-d223f7449a82