Clarify the support matrix of the cbbackupmgr tool

https://developer.couchbase.com/documentation/server/5.1/backup-restore/enterprise-backup-restore.html

We need to clarify, what is and isn't supported. From engineering, my understanding is that cbbackupmgr backups are only supported against the version it was built with. We do not support using the backup manager tool in 5.1.0 to backup a 4.6.4 cluster for example.

The restore of the backup archive itself is supported against the "major" version. So a backup taken with 5.0.0 is supported to be restored with 5.1.0.