BZ-2022831 Clarified unique domain requirements for Ingress Controller

Author: bscott-rh

modules/nw-aws-nlb-existing-cluster.adoc

@@ -50,7 +50,7 @@ spec:
           type: NLB
 ----
 <1> Replace `$my_ingress_controller` with a unique name for the Ingress Controller.
-<2> Replace `$my_unique_ingress_domain` with a domain name that is unique among all Ingress Controllers in the cluster.
+<2> Replace `$my_unique_ingress_domain` with a domain name that is unique among all Ingress Controllers in the cluster. This variable must be a subdomain of the DNS name `<clustername>.<domain>`.
 <3> You can replace `External` with `Internal` to use an internal NLB.
 
 . Create the resource in the cluster: