doc: improvements to child_process, process docs

Sort links in lexical order. Add missing links.
Add `disconnect` event description in Process doc.
Fix typos.

R-URL: https://github.com/nodejs/node/pull/5075


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Jefe Lindstädt <robert.lindstaedt@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>