document how to unmount with Hiera

admin-guide/configuration/mount.md

@@ -138,3 +138,20 @@ mounter::def::e10550:
 Note that beside the mandatory `bind` option there is
 - `_netdev` to be set when the directory to bind (`device`) is on a network volume
 - `x-systemd.requires-mounts-for=$OTHER_MOUNTPOINT` ensures that systemd prepares the bind mount after the volume on which the directory to bind (`device`) is located
+
+## Removing a Mount
+
+Only removing a mount point definition from Hiera does not unmount and remove it from the node. This can then be done manually by unmounting it and removing it from `/etc/fstab`.
+
+Alternatively an `absent` mount defintion as in below example will automatically unmount and remove the mount `/etc/fstab` entry:
+
+```
+mounter::def::scratch:
+  ensure: 'absent'
+  mountpoint: '/media/scratch'
+
+mounter::mounts:
+  - 'scratch'
+```
+
+This configuration can then be removed again after it has been rolled out once to all concernded nodes.