Inline documentation for hooks in wp-admin/network/upgrade.php.

Props miyauchi for the initial patch.
Fixes #25881.

Built from https://develop.svn.wordpress.org/trunk@26914


git-svn-id: https://core.svn.wordpress.org/trunk@26795 1a063a9b-81f0-0310-95a4-ce76da25c4cd