nixos/docker: warn about changing storageDriver

Add warning to the documentation of
`virtualisation.docker.storageDriver` that changing will cause any
existing containers and images to become inaccessible.

nixos/modules/virtualisation/docker.nix

@@ -93,8 +93,18 @@ in
         default = null;
         description =
           lib.mdDoc ''
-            This option determines which Docker storage driver to use. By default
-            it let's docker automatically choose preferred storage driver.
+            This option determines which Docker
+            [storage driver](https://docs.docker.com/storage/storagedriver/select-storage-driver/)
+            to use.
+            By default it lets docker automatically choose the preferred storage
+            driver.
+            However, it is recommended to specify a storage driver explicitly, as
+            docker's default varies over versions.
+
+            ::: {.warning}
+            Changing the storage driver will cause any existing containers
+            and images to become inaccessible.
+            :::
           '';
       };