doc: update process.send() signature

This commit brings the process.send() signature into sync with
the child_process.send() documentation. Specifically, this commit
adds the callback argument and return type to the docs.

PR-URL: https://github.com/nodejs/node/pull/5284


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>