update wording to be specific to sandbox delete ignoring removal policies

Author: rtpascual

src/pages/[platform]/build-a-backend/add-aws-services/deletion-backup-resources/index.mdx

@@ -166,7 +166,7 @@ For example, if you would like to retain a resource on stack deletion, you can u
 
 <Callout>
 
-Retaining resources on stack deletion is not supported in sandbox.
+`ampx sandbox delete` ignores any resource removal policy and always deletes all resources.
 
 </Callout>