Fixed #21875 -- Clarified that get_user_model() only works at run time.

Thanks Benjamin White for the report.

docs/releases/1.7.txt

@@ -869,6 +869,10 @@ regressions cannot be ruled out. You may encounter the following exceptions:
   will also trigger this exception. The ORM cannot function properly until all
   models are available.
 
+  Another common culprit is :func:`django.contrib.auth.get_user_model()`. Use
+  the :setting:`AUTH_USER_MODEL` setting to reference the User model at import
+  time.
+
 * ``ImportError: cannot import name ...`` This happens if the import sequence
   ends up in a loop.
 

docs/topics/auth/customizing.txt

@@ -426,6 +426,10 @@ different User model.
 
             post_save.connect(post_save_receiver, sender=settings.AUTH_USER_MODEL)
 
+    Generally speaking, you should reference the User model with the
+    :setting:`AUTH_USER_MODEL` setting in code that is executed at import
+    time. ``get_user_model()`` only works once Django has imported all models.
+
 Specifying a custom User model
 ------------------------------