Update documentation for HA to separate service HA from messages HA

The High availability section of the Broker documentation on OpenShift mixes the requirements on how to handle HA for the services (at the pod level) and the HA for the messages. using the drainer for retrieving messages when scaling down the cluster. 

This creates confusion in people reading the documentation as there is no clear separation of what applies in which case. We need to clearly separate the service from the messaging HA or not overload the term for messages.

Link: https://access.redhat.com/documentation/en-us/red_hat_amq/2020.q4/html/deploying_amq_broker_on_openshift/assembly-br-configuring-operator-based-deployments_broker-ocp#high-availability-for-broker-on-ocp_broker-ocp