[1.8.x] Fixed #24613 -- Added example to QuerySet.defer() documentation

Backport of dd99f57710 from master

docs/ref/models/querysets.txt

@@ -1379,6 +1379,30 @@ one, doing so will result in an error.
     reader, is slightly faster and consumes a little less memory in the Python
     process.
 
+    For example, both of these models use the same underlying database table::
+
+        class CommonlyUsedModel(models.Model):
+            f1 = models.CharField(max_length=10)
+
+            class Meta:
+                managed = False
+                db_table = 'app_largetable'
+
+        class ManagedModel(models.Model):
+            f1 = models.CharField(max_length=10)
+            f2 = models.CharField(max_length=10)
+
+            class Meta:
+                db_table = 'app_largetable'
+
+        # Two equivalent QuerySets:
+        CommonlyUsedModel.objects.all()
+        ManagedModel.objects.all().defer('f2')
+
+    If many fields need to be duplicated in the unmanaged model, it may be best
+    to create an abstract model with the shared fields and then have the
+    unmanaged and managed models inherit from the abstract model.
+
 .. note::
 
     When calling :meth:`~django.db.models.Model.save()` for instances with