Failover/rebalance documentation only talks about data - need to add GSI

When ns_server initiated a failover, the different services on the node will also be notified.  We need to document what happen with GSI in failover/rebalance.

For instance, if data service does not have replica enabled, then the option to failover on the node is not available because it will result in data loss.  However a rebalance on a node with index service - will allow to proceed even when the index on the node does not have replica. This will result in index loss.