Update documentation on CoreDNS (#10923)

* update documentation on CoreDNS

* Update dns-custom-nameservers.md

Fixed nit noted in comments and cleaned up a few more things while I was in there. This whole topic needs a complete redo, but this PR is an iterative improvment. Thanks!

content/en/docs/concepts/services-networking/connect-applications-service.md

@@ -122,7 +122,7 @@ about the [service proxy](/docs/concepts/services-networking/service/#virtual-ip
 
 Kubernetes supports 2 primary modes of finding a Service - environment variables
 and DNS. The former works out of the box while the latter requires the
-[kube-dns cluster addon](http://releases.k8s.io/{{< param "githubbranch" >}}/cluster/addons/dns/kube-dns/README.md).
+[CoreDNS cluster addon](http://releases.k8s.io/{{< param "githubbranch" >}}/cluster/addons/dns/coredns).
 
 ### Environment Variables
 
@@ -178,7 +178,7 @@ kube-dns   ClusterIP   10.0.0.10    <none>        53/UDP,53/TCP   8m
 
 If it isn't running, you can [enable it](http://releases.k8s.io/{{< param "githubbranch" >}}/cluster/addons/dns/README.md#how-do-i-configure-it).
 The rest of this section will assume you have a Service with a long lived IP
-(my-nginx), and a DNS server that has assigned a name to that IP (the kube-dns
+(my-nginx), and a DNS server that has assigned a name to that IP (the CoreDNS
 cluster addon), so you can talk to the Service from any pod in your cluster using
 standard methods (e.g. gethostbyname). Let's run another curl application to test this: