Add more detail to Gateway API enablement docs

Provide more guidance on enabling/disabling the Gateway API deployment controller at https://docs.openshift.com/container-platform/4.11/service_mesh/v2x/servicemesh-release-notes.html#kubernetes-gateway-api. The upstream documentation does not explain it in detail, so maybe it would be worth to say at least:

"When the Kubernetes API deployment controller is not enabled, deploy and link an ingress gateway to the created Gateway object. An ingress gateway is deployed automatically when a Gateway object is created only if the Kubernetes API deployment controller is enabled."

This is described here: https://istio.io/latest/docs/tasks/traffic-management/ingress/gateway-api/#manual-deployment

SME: jewertow@redhat.com