Fixed #17841 -- Clarified caching note about authentication backends. Thanks...

    Once a user has authenticated, Django stores which backend was used to

    authenticate the user in the user's session, and re-uses the same backend

    for subsequent authentication attempts for that user. This effectively means

    that authentication sources are cached, so if you change

    for the duration of that session whenever access to the currently

    authenticated user is needed. This effectively means that authentication

    sources are cached on a per-session basis, so if you change

    :setting:`AUTHENTICATION_BACKENDS`, you'll need to clear out session data if

    you need to force users to re-authenticate using different methods. A simple

    way to do that is simply to execute ``Session.objects.all().delete()``.