docs/zh_CN: accurate translation of "function"

Choose an accurate translation based on the context.

Signed-off-by: Lu Dai <dai.lu@exordes.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240125200549.7192-1-dai.lu@exordes.com