Use an interpolated hook name for `edit_{$post_type}_per_page` instead of `$per_page` and adjust docs accordingly.

Also references the correct variable for the `$posts_per_page` parameter in the `edit_posts_per_page` hook doc.

Props Otto42 for the initial patch.
See #26869.


git-svn-id: https://develop.svn.wordpress.org/trunk@28347 602fd350-edb4-49c9-b593-d223f7449a82