Fixed #21391 -- Allow model signals to lazily reference their senders.

docs/topics/auth/customizing.txt

@@ -413,6 +413,19 @@ different User model.
         class Article(models.Model):
             author = models.ForeignKey(settings.AUTH_USER_MODEL)
 
+    .. versionadded:: 1.7
+
+        When connecting to signals sent by the User model, you should specify the
+        custom model using the :setting:`AUTH_USER_MODEL` setting. For example::
+
+            from django.conf import settings
+            from django.db.models.signals import post_save
+
+            def post_save_receiver(signal, sender, instance, **kwargs):
+                pass
+
+            post_save.connect(post_save_receiver, sender=settings.AUTH_USER_MODEL)
+
 Specifying a custom User model
 ------------------------------