Fixed #24379 -- Documented that remote user example disables ModelBackend.

``request.META['REMOTE_USER']`` and will authenticate and auto-login that user

using the :class:`~django.contrib.auth.backends.RemoteUserBackend`.

Be aware that this particular setup disables authentication with the default

``ModelBackend``. This means that if the ``REMOTE_USER`` value is not set

then the user is unable to log in, even using Django's admin interface.

Adding ``'django.contrib.auth.backends.ModelBackend'`` to the

``AUTHENTICATION_BACKENDS`` list will use ``ModelBackend`` as a fallback

if ``REMOTE_USER`` is absent, which will solve these issues.

Django's user management, such as the views in ``contrib.admin`` and

the :djadmin:`createsuperuser` management command, doesn't integrate with

remote users. These interfaces work with users stored in the database

regardless of ``AUTHENTICATION_BACKENDS``.

.. note::

   Since the ``RemoteUserBackend`` inherits from ``ModelBackend``, you will

   still have all of the same permissions checking that is implemented in