Add more detail in Index Stats Settings

This document section provides instructions on how to change various index settings. We need to add more details.

1. Default setting values
2. The reasons & implications of changing the default values. i.e Why would user want to change maxRollbackPoints, memorySnapshotInterval,stableSnapshotInterval, etc..

https://docs.couchbase.com/server/current/rest-api/post-settings-indexes.html