docs/zh_CN: Update zh_CN/process/email-clients.rst to 5.19

* update to commit cbf4adfd ("Documentation: process: Update email
  client instructions for Thunderbird")
* clean the whole translation
Signed-off-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/3a19acf5929357f2702ac1e3538d1a9cc0085cc0.1659406843.git.bobwxc@email.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>