modules/update-service-overview: Mention support for rollbacks

Saying "there is no return, ever" might make folks nervous.  Point out
that we are there for anyone who needs help, using language from
f487886 (updates, 2019-05-21, #13993).

Author: wking

modules/update-service-overview.adoc

@@ -61,6 +61,7 @@ determine if updates are available.
 ====
 Reverting your cluster to a previous version, or a rollback, is not supported.
 Only upgrading to a newer version is supported.
+If your upgrade fails, contact Red Hat support.
 ====
 
 During the upgrade process, the Machine Config Operator (MCO) applies the new configuration to your cluster machines. It cordons the number of nodes that is specified by the `maxUnavailable` field on the machine configuration pool and marks them as unavailable. By default, this value is set to `1`. It then applies the new configuration and reboots the machine. If you use Red Hat Enterprise Linux (RHEL) machines as workers, the MCO does not update the kubelet on these machines because you must update the OpenShift API on them first. Because the specification for the new version is applied to the old kubelet, the RHEL machine cannot return to the `Ready` state. You cannot complete the update until the machines are available. However, the maximum number of nodes that are unavailable is set to ensure that normal cluster operations are likely to continue with that number of machines out of service.