Upgrade docs: reminder about custom policies

Author: MoteHue

doc/source/operations/upgrading-openstack.rst

@@ -246,6 +246,9 @@ suggestions:
 * Check Grafana dashboards.
 * Update the deployment to use the latest |previous_release| images and
   configuration.
+* If your customer has overriden any policies, check to see if they need
+  updating to align with new defaults. These will be written to files
+  ``kolla/config/<service>/policy.yaml``.
 
 RabbitMQ SLURP upgrade
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