add docs for exclude_fields_in_copy

docs/reference/pages/model_reference.rst

@@ -189,6 +189,18 @@ In addition to the model fields provided, ``Page`` has many properties and metho
 
         Controls if this page can be created through the Wagtail administration. Defaults to True, and is not inherited by subclasses. This is useful when using `multi-table inheritance <https://docs.djangoproject.com/en/1.8/topics/db/models/#multi-table-inheritance>`_, to stop the base model from being created as an actual page.
 
+    .. attribute:: exclude_fields_in_copy
+
+        An array of field names that will not be included when a Page is copied.
+        Useful when you have relations that do not use `ClusterableModel` or should not be copied.
+
+        .. code-block:: python
+
+            class BlogPage(Page):
+                exclude_fields_in_copy = ['special_relation', 'custom_uuid']
+
+        The following fields will always be excluded in a copy - `['id', 'path', 'depth', 'numchild', 'url_path', 'path']`.
+
     .. attribute:: base_form_class
 
         The form class used as a base for editing Pages of this type in the Wagtail page editor.