Fixed #9913 - Clarified User.is_authenticated docs.

Thanks rshea for the draft text.

docs/ref/contrib/auth.txt

@@ -111,10 +111,14 @@ Methods
 
     .. method:: is_authenticated()
 
-        Always returns ``True``. This is a way to tell if the user has been
-        authenticated. This does not imply any permissions, and doesn't check
-        if the user is active - it only indicates that the user has provided a
-        valid username and password.
+        Always returns ``True`` (as opposed to
+        ``AnonymousUser.is_authenticated()`` which always returns ``False``).
+        This is a way to tell if the user has been authenticated. This does not
+        imply any permissions, and doesn't check if the user is active - it
+        only indicates that ``request.user`` has been populated by the
+        :class:`~django.contrib.auth.middleware.AuthenticationMiddleware` with
+        a :class:`~django.contrib.auth.models.User` object representing the
+        currently logged-in user.
 
     .. method:: get_full_name()