Merge is not supported for cloud repos in backup service - this should be documented

Currently we are not supporting merges for cloud repo backups via backup service - we should document this properly so that the end user is aware of it.

I think it would be good to add a note or warning about the fact that merge tasks will be skipped on cloud repositories in the https://docs.couchbase.com/server/7.0/manage/manage-backup-and-restore/manage-backup-and-restore.html#schedule-merges as well as in the https://docs.couchbase.com/server/7.0/manage/manage-backup-and-restore/manage-backup-and-restore.html#use-cloud-storage.