block: Document blockdev-add's immaturity

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1426858337-21423-1-git-send-email-armbru@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

qapi/block-core.json

@@ -1721,6 +1721,10 @@
 #
 # Creates a new block device.
 #
+# This command is still a work in progress.  It doesn't support all
+# block drivers, it lacks a matching blockdev-del, and more.  Stay
+# away from it unless you want to help with its development.
+#
 # @options: block device options for the new device
 #
 # Since: 1.7

qmp-commands.hx

@@ -3636,6 +3636,10 @@ blockdev-add
 
 Add a block device.
 
+This command is still a work in progress.  It doesn't support all
+block drivers, it lacks a matching blockdev-del, and more.  Stay away
+from it unless you want to help with its development.
+
 Arguments:
 
 - "options": block driver options