[1.11.x] Fixed #27911 -- Doc'd how to register custom User with admin.

Backport of 9269dec05e from master

docs/topics/auth/customizing.txt

@@ -411,6 +411,14 @@ model, but you'll be able to customize it in the future if the need arises::
 Don't forget to point :setting:`AUTH_USER_MODEL` to it. Do this before creating
 any migrations or running ``manage.py migrate`` for the first time.
 
+Also, register the model in the app's ``admin.py``::
+
+    from django.contrib import admin
+    from django.contrib.auth.admin import UserAdmin
+    from .models import User
+
+    admin.site.register(User, UserAdmin)
+
 Changing to a custom user model mid-project
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