chore(ecs): add documentation to clarify that ECS clusters and their capacity providers

packages/aws-cdk-lib/aws-ecs/README.md

@@ -1591,6 +1591,8 @@ it in the constructor. Then add the Capacity Provider to the cluster. Finally,
 you can refer to the Provider by its name in your service's or task's Capacity
 Provider strategy.
 
+> **Note**: Cross-stack capacity provider registration is not supported. The ECS cluster and its capacity providers must be created in the same stack to avoid circular dependency issues.
+
 By default, Auto Scaling Group Capacity Providers will manage the scale-in and
 scale-out behavior of the auto scaling group based on the load your tasks put on
 the cluster, this is called [Managed Scaling](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/asg-capacity-providers.html#asg-capacity-providers-managed-scaling). If you'd