doc: add duplicate CVE check in sec. release doc

This commit adds a note about only creating a CVE for Node.js
vulnerabilities.

The motivation for this is a recent HackerOne report where I created a
CVE for a c-ares issue. This CVE should have been created by the c-ares
project, and it was later, but we never updated our HackerOne report to
use their CVE number. Hopefully this extra note in the release doc will
help us check for this situaion and avoid this in the future.

PR-URL: https://github.com/nodejs/node/pull/39845
Refs: https://hackerone.com/reports/1178337


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>