doc: add options description for send APIs

Describes the meaning of the boolean return in process.send()
(doc/api/process.md) and worker.send() (doc/api/cluster.md) as
described in subprocess.send() (doc/api/child_process.md)

Fixes: https://github.com/nodejs/node/issues/26995
PR-URL: https://github.com/nodejs/node/pull/29868


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>