Add upgrades doc note (#526)

* Add upgrades doc note

Add a note so that upgrade testing (charms and payload)
remains in sync with the end-user documentation (CDG)

zaza/openstack/utilities/upgrade_utils.py

@@ -26,7 +26,13 @@ from zaza.openstack.utilities.os_versions import (
     OPENSTACK_RELEASES_PAIRS,
 )
 
-
+"""
+The below upgrade order is surfaced in end-user documentation. Any change to
+it should be accompanied by an update to the OpenStack Charms Deployment Guide
+for both charm upgrades and payload upgrades:
+- source/upgrade-charms.rst#upgrade-order
+- source/upgrade-openstack.rst#openstack_upgrade_order
+"""
 SERVICE_GROUPS = (
     ('Database Services', ['percona-cluster', 'mysql-innodb-cluster']),
     ('Stateful Services', ['rabbitmq-server', 'ceph-mon']),