Add a snippet of documentation for a m.push_rules event.

specification/modules/push.rst

@@ -336,6 +336,14 @@ using the APIs below.
 
 {{pushrules_http_api}}
 
+
+Push Rules: Events
+~~~~~~~~~~~~~~~~~~
+
+A user changes their push rules a ``m.push_rules`` event is sent to all clients
+in the ``account_data`` section of their next /sync request. The content of the
+event is the current push rules for the user.
+
 Examples
 ++++++++