Rename the automatic_updates_send_email filter to auto_core_update_send_email. (Final name change.)

fixes #25653. Also:
 * Fully document all new hooks, methods, and classes in the upgrader.
 * Rename 'language' to 'translation' inside the upgrader.
 * Improve the readability of the crazy do-while loop in the is_vcs_checkout() method.


git-svn-id: https://develop.svn.wordpress.org/trunk@25859 602fd350-edb4-49c9-b593-d223f7449a82