[1.6.x] Fixed #23725 -- Substituted AUTH_USER_MODEL for User in docs.

Backport of 68d3db8a7b from master

docs/ref/models/fields.txt

@@ -1174,14 +1174,16 @@ The possible values for :attr:`~ForeignKey.on_delete` are found in
     necessary to avoid executing queries at the time your models.py is
     imported::
 
+        from django.conf import settings
+        from django.contrib.auth import get_user_model
         from django.db import models
-        from django.contrib.auth.models import User
 
         def get_sentinel_user():
-            return User.objects.get_or_create(username='deleted')[0]
+            return get_user_model().objects.get_or_create(username='deleted')[0]
 
         class MyModel(models.Model):
-            user = models.ForeignKey(User, on_delete=models.SET(get_sentinel_user))
+            user = models.ForeignKey(settings.AUTH_USER_MODEL,
+                                     on_delete=models.SET(get_sentinel_user))
 
 * .. attribute:: DO_NOTHING
 
@@ -1346,12 +1348,12 @@ as default value.
 
 With the following example::
 
+    from django.conf import settings
     from django.db import models
-    from django.contrib.auth.models import User
 
     class MySpecialUser(models.Model):
-        user = models.OneToOneField(User)
-        supervisor = models.OneToOneField(User, related_name='supervisor_of')
+        user = models.OneToOneField(settings.AUTH_USER_MODEL)
+        supervisor = models.OneToOneField(settings.AUTH_USER_MODEL, related_name='supervisor_of')
 
 your resulting ``User`` model will have the following attributes::