Fixed #27352 -- Doc'd social media fingerprinting consideration with login's redirect_authenticated_user.

docs/spelling_wordlist

@@ -253,6 +253,7 @@ fallback
 fallbacks
 faq
 FastCGI
+favicon
 fieldset
 fieldsets
 filename

docs/topics/auth/default.txt

@@ -1006,6 +1006,15 @@ implementation details see :ref:`using-the-views`.
       authenticated users accessing the login page will be redirected as if
       they had just successfully logged in. Defaults to ``False``.
 
+      .. warning::
+
+        If you enable ``redirect_authenticated_user``, other websites will be
+        able to determine if their visitors are authenticated on your site by
+        requesting redirect URLs to image files on your website. To avoid
+        this "`social media fingerprinting
+        <https://robinlinus.github.io/socialmedia-leak/>`_" information
+        leakage, host all images and your favicon on a separate domain.
+
     * ``success_url_allowed_hosts``: A :class:`set` of hosts, in addition to
       :meth:`request.get_host() <django.http.HttpRequest.get_host>`, that are
       safe for redirecting after login. Defaults to an empty :class:`set`.