Expand a little on the specifics of backups of an LVM volume (#1014).

Reviewed-on: https://projects.torsion.org/borgmatic-collective/borgmatic/pulls/1014

docs/how-to/snapshot-your-filesystems.md

@@ -205,6 +205,14 @@ Volume Manager) and sending those snapshots to Borg for backup. LVM isn't
 itself a filesystem, but it can take snapshots at the layer right below your
 filesystem.
 
+Note that, due to borg being a file-level backup, this feature is really only
+suitable for filesystems, not whole disk or raw images containing multiple
+filesystems (for example, if you're using a LVM volume to run a Windows
+KVM that contains an MBR, partitions, etc.)
+
+In those cases, you can omit the `lvm:` option, and use borg's own support for
+[image backup](https://borgbackup.readthedocs.io/en/stable/deployment/image-backup.html).
+
 To use this feature, first you need one or more mounted LVM logical volumes.
 Then, enable LVM within borgmatic by adding the following line to your
 configuration file: