Inline documentation for various SQL clause hooks in wp-includes/query.php.

Covers documentation for SQL clause hooks related to comment feeds, including:
* `comment_feed_join`
* `comment_feed_where`
* `comment_feed_groupby`
* `comment_feed_orderby`
* `comment_feed_limits`

Props dougwollison, DrewAPicture.
See #25514.


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