Merge pull request #3840 from harveyxia/comment-specific-envsubst-usage

docs: restore comment describing specific envsubst usage

Author: k8s-ci-robot

config/rbac/serviceaccount.yaml

@@ -6,4 +6,6 @@ metadata:
   labels:
     control-plane: controller-manager
   annotations:
+    # The following uses the prefix substitution functionality of envsubst (https://github.com/drone/envsubst)
+    # Not compatible with GNU envsubst
     ${AWS_CONTROLLER_IAM_ROLE/#arn/eks.amazonaws.com/role-arn: arn}