Fixed #19218 -- Added documentation note on limitations of signals with custom User models.

Thanks to kunitoki@gmail.com for the report.

docs/topics/auth.txt

@@ -2148,6 +2148,13 @@ If your project uses proxy models, you must either modify the proxy to extend
 the User model that is currently in use in your project, or merge your proxy's
 behavior into your User subclass.
 
+Custom users and signals
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Another limitation of custom User models is that you can't use
+:func:`django.contrib.auth.get_user_model()` as the sender or target of a signal
+handler. Instead, you must register the handler with the actual User model.
+
 A full example
 --------------