doc: revise handling-own-pull-requests text

Simplify and clarify the text of the COLLABORATORS_GUIDE where handling
one's own pull requests is concerned.

PR-URL: https://github.com/nodejs/node/pull/24583


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>