chore(eks): document failure conditions for albController (#28419)

`albController` would fail the deployment if there's no defaultCapacity or nodegroup. This PR improves the doc about it.

Documents issue from #22005 but further effort is needed to create a synth-time error.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

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

@@ -615,6 +615,9 @@ new eks.Cluster(this, 'HelloEKS', {
 });
 ```
 
+The `albController` requires `defaultCapacity` or at least one nodegroup. If there's no `defaultCapacity` or available
+nodegroup for the cluster, the `albController` deployment would fail.
+
 Querying the controller pods should look something like this:
 
 ```console