doc: revise premature disclosure text in collaborator guide

Avoid abbreviations and jargon. PR -> pull request, repo -> repository,
etc.

PR-URL: https://github.com/nodejs/node/pull/37524


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>