Add reference to push.enabled to homeserver.yaml

Related to:

- https://github.com/matrix-org/synapse/pull/14551/files
- https://github.com/matrix-org/synapse/pull/14619/files

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -2532,6 +2532,8 @@ password_providers:
 ## Push ##
 
 push:
+  # enabled: false
+
   # Clients requesting push notifications can either have the body of
   # the message sent in the notification poke along with other details
   # like the sender, or just the event ID and room ID (`event_id_only`).