child_process: document kill() return value

This commit documents the return value from subprocess.kill().

PR-URL: https://github.com/nodejs/node/pull/30669
Refs: https://github.com/nodejs/node/issues/30668


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>