[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). From the documentation, it's not really clear that a backup CR might have only local snapshot or off-system backup configured at any time.
If anyone needs both, two different backup CRs must be created aiming to backup the same content using different methods.

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/backup_and_restore/application-backup-and-restore#annotations:f3595cca-51fb-4d4d-be6b-f7a04a29d2fe