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

Covers documentation for general SQL clause hooks, including:
* `posts_where`
* `posts_join`
* `posts_where_paged`
* `posts_groupby`
* `posts_join_paged`
* `posts_orderby`
* `posts_distinct`
* `post_limits`
* `posts_fields`
* `posts_clauses`

Props dougwollison, DrewAPicture.
See #25514.



git-svn-id: https://develop.svn.wordpress.org/trunk@27206 602fd350-edb4-49c9-b593-d223f7449a82