feat: Add instructional note to Swagger documentation (#3532)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

src/plugins/api-server/backend/main.ts

@@ -120,6 +120,7 @@ export const backend = createBackend<BackendType, APIServerConfig>({
       info: {
         version: '1.0.0',
         title: 'Youtube Music API Server',
+        description: 'Note: You need to get an access token using the `/auth/{id}` endpoint first to call any API endpoints under `/api`.',
       },
       security: [
         {