Be clear that rabbitmq-upgrade is not idempotent

This isn't obvious from reading of the docs, let's make sure users are
aware.

Change-Id: I0da1ad16bfab00bed7e935cca118551a6c5860c6

doc/source/reference/message-queues/rabbitmq.rst

@@ -180,6 +180,9 @@ therefore RabbitMQ version 3.13).
 
    This command should be run from the Antelope release.
 
+   Note that this command is NOT idempotent. See "RabbitMQ versions" below for
+   an alternative approach.
+
 .. code-block:: console
 
    kolla-ansible rabbitmq-upgrade 3.12