Document the new backupPVC/restorePVC secretNames and configMapNames
options (velero#9920) that copy namespace-scoped secrets/configmaps to
the Velero namespace so datamover can back up and restore encrypted CSI
volumes (e.g. ODF/ceph-csi with Vault KMS). Updates the main and v1.18
docs.
Signed-off-by: Shubham Pampattiwar <spampatt@redhat.com>