doc: clarify what moderation issues are for

PR-URL: nodejs#51990
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

onboarding.md

@@ -102,7 +102,10 @@ The project has a venue for real-time discussion:
 
 * When a discussion gets heated, you can request that other collaborators keep
   an eye on it by opening an issue at the private
-  [nodejs/moderation](https://github.com/nodejs/moderation) repository.
+  [nodejs/moderation](https://github.com/nodejs/moderation) repository. Note
+  that while that repository is not public, it can be accessed by anyone in the
+  nodejs org, so refrain from using it to report individuals (reporting
+  spam/bots there is fine of course).
   * This is a repository to which all members of the `nodejs` GitHub
     organization (not just collaborators on Node.js core) have access. Its
     contents should not be shared externally.