documentation for GSI restore

The restore process of GSI indexes need to be documented:

1. After restore, the indexes do not get built automatically. This allows the user to
   1. verify if the desired indexes have been restored
   2. change the placement if required(using alter index) and then depend
   3. build indexes in batches depending on how many resources are available in the cluster.
2. User can build multiple indexes using a single command using the new syntax provided in DOC-4596
3. Backups can be restored to a different destination bucket. This has been fixed in MH MB-29843