Change trigger spec to indicate assigned broker should be immutable
@@ -34,8 +34,9 @@ subscriber.
While a Trigger is Ready, it SHOULD indicate its subscriber's URI via the
`status.subscriberUri` field.
-Triggers MUST be assigned to exactly one Broker. Triggers SHOULD be assigned a
-default Broker upon creation if no Broker is specified by the user.
+Triggers MUST be assigned to exactly one Broker. The assigned Broker of a
+trigger SHOULD be immutable. Triggers SHOULD be assigned a default Broker
+upon creation if no Broker is specified by the user.
A Trigger MAY be created before its assigned Broker exists. A Trigger SHOULD
progress to Ready when its assigned Broker exists and is Ready.
