In commit 50243407 "qapi/qom: Drop deprecated 'props' from
object-add" (v6.0.0), we also should update documents.

Signed-off-by: Lei Rao <lei.rao@intel.com>
Message-Id: <1637567387-28250-1-git-send-email-lei.rao@intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>