doc: document use of Refs: for references

Standardise on Refs:

PR-URL: https://github.com/nodejs/node/pull/10670


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>