docs: use explicit register_delete_rule example

The previous example of creating bi-directional delete rules was vague since the example defined only one class and the relationship between "Foo" and "Bar" wasn't clear. I added a more explicit example where the relationship between the two classes is explicit.

mongoengine/fields.py

@@ -971,11 +971,13 @@ class ReferenceField(BaseField):
 
     .. code-block:: python
 
-        class Bar(Document):
+        class Org(Document):
-            content = StringField()
+            owner = ReferenceField('User')
-            foo = ReferenceField('Foo')
 
-        Foo.register_delete_rule(Bar, 'foo', NULLIFY)
+        class User(Document):
+            org = ReferenceField('Org', reverse_delete_rule=CASCADE)
+
+        User.register_delete_rule(Org, 'owner', DENY)
 
     .. versionchanged:: 0.5 added `reverse_delete_rule`
     """