Add comment about service channels for updating

app/celery/broadcast_message_tasks.py

@@ -75,6 +75,12 @@ def send_broadcast_provider_message(broadcast_event_id, provider):
             previous_provider_messages=broadcast_event.get_earlier_provider_messages(provider),
             sent=broadcast_event.sent_at_as_cap_datetime_string,
             expires=broadcast_event.transmitted_finishes_at_as_cap_datetime_string,
+            # We think an alert update should always go out on the same channel that created the alert
+            # We recognise there is a small risk with this code here that if the services channel was
+            # changed between an alert being sent out and then updated, then something might go wrong
+            # but we are relying on service channels changing almost never, and not mid incident
+            # We may consider in the future, changing this such that we store the channel a broadcast was
+            # sent on on the broadcast message itself and pick the value from there instead of the service
             channel=channel
         )
     elif broadcast_event.message_type == BroadcastEventMessageType.CANCEL: