Remove redirect loop warning re. using redirect_authenticated_user with permissions checking.

[brickl]

As per ​redirecting-unauthorized-requests-in-class-based-views:

Changed in Django 2.1:
In older versions, authenticated users who lacked permissions were redirected to the login page (which resulted in a loop) instead of receiving an HTTP 403 Forbidden response.

However, there is still a warning under ​all-authentication-views that states that "Enabling redirect_authenticated_user can also result in a redirect loop when using the permission_required() decorator unless the raise_exception parameter is used." This warning no longer seems to be relevant given the change described above, and should be deleted.

[Tim Graham]

I think the note is still relevant. It was added in df90e462d91d3a77aa89b69d791bf17c2bf7ff9b (#29212) along with tests. The code change you mentioned was merged earlier in 9b1125bfc7e2dc747128e6e7e8a2259ff1a7d39f (#28379).

[brickl]

My apologies, I conflated the permission_required decorator with the PermissionRequiredMixin. I thought that the raise_exception parameter of the decorator and the raise_exception attribute of the mixin worked the same way, but they do not (this distinction does seem to create unnecessary confusion though).