Fixes #20244 -- Recommend swappable users are named User.

Swappable users which are using `PermissionsMixin` should be called `User` wherever possible to ensure maximum swappability. We cannot fix the related_name of the relationships defined in `PermissionsMixin` as it would break backwards compatibility. This should be considered an optional extension to the user contract for external applications if they are depending on those automatically generated relationships.

Ticket: https://code.djangoproject.com/ticket/20244

I've also updated all examples of `MyUser` in the documentation to be called `User` to set a better example.