adding note on retention for deleted apps

Author: btardif

articles/app-service/app-service-undelete.md

@@ -11,6 +11,10 @@ ms.topic: article
 
 If you happened to accidentally delete your app in Azure App Service, you can restore it using the commands from the [Az PowerShell module](https://docs.microsoft.com/powershell/azure/?view=azps-2.6.0&viewFallbackFrom=azps-2.2.0).
 
+> [!NOTE]
+> Deleted apps are purged from the system 30 days after the initial deletion. Once an app has been purged, it can't be recovered.
+>
+
 ## Re-register App Service resource provider
 Some customers might encounter an issue where retrieving the list of deleted apps fails. To resolve the issue, run the following command: