doc: add `added:` information for process

Most of the information is gleaned from changelogs. The rest is inferred
from git history.

Omitted cpuUsage() since it's not yet in a release.

Also omitted the streams, events and signals, since I didn't really
think it made sense for those.

Ref: https://github.com/nodejs/node/issues/6578
PR-URL: https://github.com/nodejs/node/pull/6589


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Alexander Makarenko <estliberitas@gmail.com>