doc: document 'unhandledRejection' and 'rejectionHandled'

Documents the new unhandled rejection detection API.

Documents the new unhandledRejection/rejectionHandled events in the process
docuemntation. As agreed on in this issue:

https://github.com/iojs/io.js/issues/256#event-241385784

PR-URL: https://github.com/iojs/io.js/pull/946


Reviewed-By: Domenic Denicola <domenic@domenicdenicola.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>