Operational concerns for Replication should call out ZooKeeper

Our design invariants state that ZooKeeper data is safe to delete. However, replication only stores its data in zookeeper.
This can lead to operators accidentally disabling their replication set up while attempting to recover from an unrelated issue by clearing the zk state.

We should update the operational concerns section on replication to call out that the /hbase/replication tree should not be deleted. We should probably also add a warning to the set up steps.