docs(nextjs): Add alert to cron docs (#11334)

Author: chargome

platform-includes/crons/setup/javascript.nextjs.mdx

@@ -7,6 +7,10 @@ Instrumented cron jobs are decided at runtime by examining the `crons` field in
 
 Set the `automaticVercelMonitors` option to `true` in your Sentry settings in `next.config.js`. See <PlatformLink to="/manual-setup/#opt-out-of-auto-instrumentation-of-vercel-cron-jobs">Manual Setup</PlatformLink> for more details.
 
+<Alert level="warning">
+Automatic instrumentation of Vercel cron jobs currently only works for the Pages Router. App Router route handlers are not yet supported.
+</Alert>
+
 ## Automatic Crons Instrumentation
 
 <Include name="javascript-crons-automatic-crons-instrumentation.mdx" />