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

Covers documentation for SQL clause hooks specified for use by caching plugins, including:
* `posts_selection`
* `posts_where_request`
* `posts_groupby_request`
* `posts_join_request`
* `posts_orderby_request`
* `posts_distinct_request`
* `posts_fields_request`
* `post_limits_request`
* `posts_clauses_request`

Props dougwollison, DrewAPicture.
See #25514.


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