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.



git-svn-id: https://develop.svn.wordpress.org/trunk@27207 602fd350-edb4-49c9-b593-d223f7449a82
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.



git-svn-id: https://develop.svn.wordpress.org/trunk@27208 602fd350-edb4-49c9-b593-d223f7449a82
Allow pseudo post types `attachment:audio` and `attachment:video` to get the media modal on Edit Media when they support featured images.

Introduces `post_supports_thumbnails( $post )` and `theme_supports_thumbnails( $post )` to cut down on duplicated code everytime this needs to be checked. There will be more cases forthcoming.

See #26631.



git-svn-id: https://develop.svn.wordpress.org/trunk@27209 602fd350-edb4-49c9-b593-d223f7449a82