[mount] Add documentation to the config file

src/modules/mount/mount.conf

@@ -42,12 +42,21 @@ extraMountsEfi:
       mountPoint: /sys/firmware/efi/efivars
 
 # Btrfs subvolumes to create if root filesystem is on btrfs volume.
-# If mountpoint is mounted already to another partition, it is ignored.
+# If *mountpoint* is mounted already to another partition, it is ignored.
 # Separate subvolume for swapfile is handled separately and automatically.
+#
+# It is possible to prevent subvolume creation -- this is likely only relevant
+# for the root (/) subvolume -- by giving an empty string as a subvolume
+# name. In this case no subvolume will be created. When using snapper as
+# a rollback mechanism, it is recommended to **not** create a subvolume
+# for root.
 
 btrfsSubvolumes:
     - mountPoint: /
       subvolume: /@
+      # As an alternative:
+      #
+      # subvolume: ""
     - mountPoint: /home
       subvolume: /@home
     - mountPoint: /var/cache