doc: improve the instructions of onboarding PR

PR-URL: https://github.com/nodejs/node/pull/18268
Fixes: https://github.com/nodejs/node/issues/18090
Refs: https://github.com/nodejs/TSC/pull/472


Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>