qemu-img: document 'info --backing-chain'

Signed-off-by: Kashyap Chamarthy <kashyap.cv@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 0ef82e9..a181363 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -34,9 +34,9 @@
 ETEXI
 
 DEF("info", img_info,
-    "info [-f fmt] [--output=ofmt] filename")
+    "info [-f fmt] [--output=ofmt] [--backing-chain] filename")
 STEXI
-@item info [-f @var{fmt}] [--output=@var{ofmt}] @var{filename}
+@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename}
 ETEXI
 
 DEF("snapshot", img_snapshot,