Expanded example of using ListFields for one-to-many relationships.

- Added an example of how to use the atomic update operations to
  pull and push to a ListField containing ReferenceFields.

docs/guide/defining-documents.rst

@@ -287,6 +287,12 @@ instance of the object to the query::
     # Find all pages that both Bob and John have authored
     Page.objects(authors__all=[bob, john])
 
+    # Remove Bob from the authors for a page.
+    Page.objects(id='...').update_one(pull__authors=bob)
+
+    # Add John to the authors for a page.
+    Page.objects(id='...').update_one(push__authors=john)
+
 
 Dealing with deletion of referred documents
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