[Docs] Hint that update() needs to eb called

docs/dev/01_live_document_types.rst

@@ -46,3 +46,8 @@ can instead override the ``update`` methodd:
 	   # Do whatever is necessary to get file contents
 
 	   self.current_file.save(self.filename, content)
+
+You need to ensure that ``update()`` is called whenever you
+need to provide a new version of your document. One possibility
+is to listen to some relevant DJango signal, then call ``update()``
+if necessary.