[1.7.x] Fixed #23847 -- Improved the email_check example in the auth documentation.

Backport of 87bd13617c57a806deb550e3af1c06ce00517a9c from master.
[1.7.x] Fixed typo in docs/topics/forms/modelforms.txt.

Backport of a15aba66f2fc650f70a791fb7fa880cf85cb1229 from master
[1.7.x] Added notes on registering signals in ready() and using dispatch_uid.

Refs #23641.

Backport of d66bda60590daabe21f60a532a613a31a10fedbd from master