Fixed #19394 --Added note about auth forms and custom user models.

docs/topics/auth/customizing.txt

@@ -674,6 +674,8 @@ custom profile fields. This class provides the full implementation of the
 default :class:`~django.contrib.auth.models.User` as an :ref:`abstract model
 <abstract-base-classes>`.
 
+.. _custom-users-and-the-built-in-auth-forms:
+
 Custom users and the built-in auth forms
 ----------------------------------------
 

docs/topics/auth/default.txt

@@ -926,6 +926,14 @@ If you don't want to use the built-in views, but want the convenience of not
 having to write forms for this functionality, the authentication system
 provides several built-in forms located in :mod:`django.contrib.auth.forms`:
 
+.. note::
+    The built-in authentication forms make certain assumptions about the user
+    model that they are working with. If you're using a :ref:`custom User model
+    <auth-custom-user>`, it may be necessary to define your own forms for the
+    authentication system. For more information, refer to the documentation
+    about :ref:`using the built-in authentication forms with custom user models
+    <custom-users-and-the-built-in-auth-forms>`.
+
 .. class:: AdminPasswordChangeForm
 
     A form used in the admin interface to change a user's password.