since f1d3e586, the code had changed the default value to 'off', so this patch
make document and code are consistent.
Signed-off-by: NLi Zhijian <lizhijian@cn.fujitsu.com>
Message-id: 1470024419-10886-1-git-send-email-lizhijian@cn.fujitsu.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>