Add backup info to RStor page

On the rstor info page at https://docs.unity.rc.umass.edu/about/rstor , after "Backups (if applicable) are retained for 90 days and performed daily from snapshots that are taken every 4 hours. 18 snapshots are retained and can be accessed at /.zfs/snapshot but user self-restore functionality from this location hasn’t been extensively tested yet" add "Backups are not currently offsite or immutable."