Add a link to the push module in the push gateway spec

changelogs/push_gateway/newsfragments/2763.clarification

@@ -0,0 +1 @@
+Clarify where to get information about the various parameter values for the notify endpoint.

specification/push_gateway.rst

@@ -89,4 +89,7 @@ the event-related fields in the ``/notify`` request. When the homeserver is perf
 a push where the ``format`` is ``"event_id_only"``, only the ``event_id``, ``room_id``,
 ``counts``, and ``devices`` are required to be populated.
 
+Note that most of the values and behaviour of this endpoint is described by the Client-Server
+API's `Push Module <../client_server/%CLIENT_RELEASE_LABEL%.html#module-push>`_.
+
 {{push_notifier_push_http_api}}