diff --git a/server/src/modules/registries.js b/server/src/modules/registries.js
index cb96a0e..828cd9d 100644
--- a/server/src/modules/registries.js
+++ b/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)