doc: revise accepting-modifications in guide

Revise the "Accepting Modifications" section of the Collaborator Guide.

PR-URL: https://github.com/nodejs/node/pull/24650


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>