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>

doc/api/process.markdown

@@ -750,6 +750,8 @@ relied upon to exist.
 
 * `message` {Object}
 * `sendHandle` {Handle object}
+* `callback` {Function}
+* Return: {Boolean}
 
 When Node.js is spawned with an IPC channel attached, it can send messages to its
 parent process using `process.send()`. Each will be received as a