doc: improve onboarding-extras.md formatting

Fixes some formatting, improves some formatting, updates minor nits.

Refs: https://github.com/nodejs/node/issues/6655
PR-URL: https://github.com/nodejs/node/pull/6548


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>