FTS - Improve rebalance/failover documentation for the search service.

Need to give more context about the service internals/impact during rebalance. 

-how the partitions are handled during a rebalance

-any impacts on the live traffic updates.

-tips for faster rebalance operations.

how to enable the quick recovery of partitions during failover upgrade - recovery of a node.

-how to deal with rebalance failures.

-Is it safe/recommended to retry a failed rebalance.

-things to keep in mind before doing a rebalance.

-things to keep keep track of during a rebalance.