[Doc][RFE] Add clarification that only the snapshot or offsite backup can be configured per backup CR

As a user, perhaps the intention might be to have both, local snapshots and off-system backup (.e.g Restic to S3). The statement, however, might not make it clear that only the one or the other can be used within a single backup CR.

Also, a hint that for two different goals to accomplish, local snapshot and off-system backup, one would need to create a separate backup CRs.

Also, it might be added that snapshot based backup will remain local to the system and is not a full backup for restoring the content to another system, perhaps in case that the local system is unavailable.

Reported by: matthiasmuench

https://access.redhat.com/documentation/en-us/openshift_container_platform/4.12/html/virtualization/backup-and-restore#annotations:d0382db3-9622-4400-8c8d-cc5585e95da2