doc: edit COLLABORATOR_GUIDE.md on closing issues

Make the COLLABORATOR_GUIDE.md text on closing issues clearer. (Make it
clear that all the text refers to issues *and* pull requests, etc.)

PR-URL: https://github.com/nodejs/node/pull/24477


Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>