Inline documentation for hooks in wp-signup.php.

Props kpdesign, miyauchi.

Fixes #25381.

Built from https://develop.svn.wordpress.org/trunk@25645


git-svn-id: http://core.svn.wordpress.org/trunk@25562 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Merge branch 'master' of https://github.com/WordPress/WordPress