docs(modules): note that registerExtension's spec-file argument is core-only

Co-Authored-By: Claude <noreply@anthropic.com>

server/src/modules/registries.js

@@ -301,6 +301,8 @@ function registerCore() {
   // whole claim PR 4 is making.
   api.registerNotificationStreams(shardStreams.STREAMS)
   api.registerAnnounceLeg(townCrierLeg.leg)
+  // The third argument is core-only and has no module counterpart — see
+  // checkExtensionShape. A module ships a prebuilt swagger-fragment.json instead.
   api.registerExtension('admin.users.detail', shardExtension, require.resolve('../router/v1/admin/usersShard.router'))
 
   apply(api.staged)