[Documentation] Document correct GFS2 setup for shared store

Hi, as SAN build on GFS2 is one of the supported storages in shared store ha topology, we should provide information how this network mount should be configured so it provides reliable failover/failback background for ActiveMQ Artemis.

I can direct you to dprezhev@redhat.com who is one of our sys-ops guys. He did some network storage setup in our messaging-lab before and it seems to be working fine.