doc: revise deprecation level explanations in Collaborator Guide

Revise deprecation level explanations for scanability and ease of
understanding.

PR-URL: https://github.com/nodejs/node/pull/26197


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>