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: nodejs#6578
PR-URL: nodejs#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>