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.


git-svn-id: https://develop.svn.wordpress.org/trunk@27667 602fd350-edb4-49c9-b593-d223f7449a82