Updated man page.

Signed-off-by: Cristiano Urban <cristiano.urban@inaf.it>

client/vos_storage

@@ -202,6 +202,9 @@ DESCRIPTION
        Adding 'hot' or 'cold' storage points requires a base path
        to be specified (e.g. '/mnt/my_storage/users').
 
+       Adding a 'cold' storage point requires selecting a tape pool
+       (a tape pool list is provided).
+
        All storage points require a valid hostname or IP address.
     """)