docs: Improve DNS port documentation
docs: Improve DNS port documentation

Some users had expressed confusion when using non-standard ports in
conjunction with DNS policy. Clarify that when there is a k8s service,
the CoreDNS / kube-dns port must be the backend port.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

tgraf/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium