Clarify meaning of repeat_interval setting in Alertmanager config

Update the sample code in the following topic to include the definition of the repeat_interval parameter.

https://docs.openshift.com/container-platform/4.11/monitoring/managing-alerts.html#applying-custom-alertmanager-configuration_managing-alerts

Per a comment form spasquie@redhat.com in the original issue linked below: "The semantics are that the notification will be repeated after at least 5 minutes but the exact timing is nondeterministic. Concretely the repeated notification should happen somewhere between (N * group_interval) and (N+1) * group_interval, with N being the integer division of repeat_interval by group_interval. If you want the notification to be repeated at each group interval, repeat_interval should to be less than group_interval. But the repeated notification might still be delayed when some Alertmanager pods are restarted/rescheduled for instance."