Inline documentation for hooks in wp-admin/includes/post.php.

Also moves the primary PHPDoc for the `edit_posts_per_page` hook to wp-admin/includes/post.php.

Props kpdesign for some cleanup.
Fixes #27431.

Built from https://develop.svn.wordpress.org/trunk@27667


git-svn-id: http://core.svn.wordpress.org/trunk@27510 1a063a9b-81f0-0310-95a4-ce76da25c4cd