doc: update onboarding PR landing info

Clarify a few items in the onboarding doc about landing a PR. One
addition is to include the optional `Refs:` metadata.

PR-URL: https://github.com/nodejs/node/pull/8479


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>