Inline documentation for three remaining hooks in wp-includes/general-template.php.

Documents the `login_form_top`, `login_form_middle`, and `login_form_bottom` and filters.

That's 184/184 hook docs files complete. Boom!

Props kpdesign, DrewAPicture.
Fixes #27719.


git-svn-id: https://develop.svn.wordpress.org/trunk@28131 602fd350-edb4-49c9-b593-d223f7449a82