doc/zh_CN: add clearing-warn-once Chinese version

and link it into admin-guide.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Harry Wei <harryxiyou@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20200713115807.56362-2-alex.shi@linux.alibaba.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>