Docs: mention ACM in Certificate Discovery

Author: sdarwin

docs/guide/ingress/cert_discovery.md

@@ -1,7 +1,7 @@
 # Certificate Discovery
 TLS certificates for ALB Listeners can be automatically discovered with hostnames from Ingress resources if the [`spec.certificateArn`](ingress_class.md#speccertificatearn) in [`IngressClassParams`](ingress_class.md#ingressclassparams-specification) or [`alb.ingress.kubernetes.io/certificate-arn`](annotations.md#certificate-arn) annotation is not specified.
 
-The controller will attempt to discover TLS certificates from the `tls` field in Ingress and `host` field in Ingress rules.
+The controller will attempt to discover TLS certificates from ACM, based on the `tls` field in Ingress and `host` field in Ingress rules.
 
 !!!note ""
     You need to explicitly specify to use HTTPS listener with [listen-ports](annotations.md#listen-ports) annotation.