doc: do not advise to cancel full CI on onboarding

Refs: https://github.com/nodejs/node/pull/21938#discussion_r204257281
and discussion below the comment.

PR-URL: https://github.com/nodejs/node/pull/21977


Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>