virsh: Be explicit that vol-resize is an offline operation.

It is stated in the manual already, so let's mention the same thing in
the description to avoid causing problems.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

tools/virsh-volume.c

@@ -1050,7 +1050,9 @@ static const vshCmdInfo info_vol_resize[] = {
      .data = N_("resize a vol")
     },
     {.name = "desc",
-     .data = N_("Resizes a storage volume.")
+     .data = N_("Resizes a storage volume. This is safe only for storage "
+                "volumes not in use by an active guest.\n"
+                "See blockresize for live resizing.")
     },
     {.name = NULL}
 };