doc: specify cluster worker.kill() caveat

worker.kill() relies on a graceful disconnect, which might not
always be possible. This commit calls this out in the docs, and
specifies worker.process.kill() as a non-graceful alternative.

PR-URL: https://github.com/nodejs/node/pull/23165
Fixes: https://github.com/nodejs/node/issues/22703


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>