Coding Standards: Use strict type check for `in_array()` in `wp-includes/feed.php`.

Additionally:
* Correct inline comments per the documentation standards.
* Correct the `@ticket` reference in `tests/feed/atom.php`.

Follow-up to [48429].

See #33591.
Built from https://develop.svn.wordpress.org/trunk@48435


git-svn-id: https://core.svn.wordpress.org/trunk@48204 1a063a9b-81f0-0310-95a4-ce76da25c4cd