Specify behaviour of vg-name change in gnt-cluster manual

The current behaviour of a volume group name change after the cluster
has been initialized can be somewhat unclear. This patch specifies it
more esplicitly in the man page.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

man/gnt-cluster.rst

@@ -217,7 +217,11 @@ The ``--vg-name`` option will let you specify a volume group
 different than "xenvg" for Ganeti to use when creating instance
 disks. This volume group must have the same name on all nodes. Once
 the cluster is initialized this can be altered by using the
-**modify** command. If you don't want to use lvm storage at all use
+**modify** command. Note that if the volume group is modified after
+the cluster creation, the metavg will have to be manually modified
+accordingly.
+
+If you don't want to use lvm storage at all use
 the ``--no-lvm-storage`` option. Once the cluster is initialized
 you can change this setup with the **modify** command.