docs/zh_CN: add completion translation

Translate .../scheduler/completion.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Acked-by: Tang Yizhou <tangyizhou@huawei.com>
Link: https://lore.kernel.org/r/47b6fd1f277058dfb8d06dd3d33fedc2ef55fcbb.1638777809.git.siyanteng@loongson.cn


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