Change the default instance name to tier-replicaset_no-isntance_no_in_replicaset and document it

Summary

Breaking change: new pattern to generate instance_name and replicaset_name

replicaset_name = {tier_name}_{replicaset_number_in_this_tier}

instance_name = {tier_name}_{replicaset_number}_{instance_number_in_replicaset}

• Close #1122 (closed)
• Cherry-pick to: none
• Docs follow-up: docs#437 (closed)
• Changelog is updated

Closes #1122 (closed)