doc: revise internal vs. public API in Collaborator Guide

Make the Collaborator Guide section "Internal vs. Public API" more
concise and clear. Remove repetition. Clarify some ambiguity.
wordy, repetitive, and somewhat

PR-URL: https://github.com/nodejs/node/pull/24975


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>