Correct misleading wording in Pod DNS Policies

A debugging rabbit hole lead me to digging around kublet's behavior
regarding certain DNS Policies.

This commit updates some potentially misleading wordings to more
correctly convey kubelet's behavior which was discovered after some
rabbit hole debugging of DNS Policies.

The ClusterFirst policy documentation would imply that the resolve.conf
of a Pod would contain a fallback to the Node's nameserver. This is not
the cause, [kubelet explicitly
mentions](4276ed3628/pkg/kubelet/network/dns/dns.go (L403-L407))
that the fallback is handled by whatever system is running kube-dns or
coredns which dramatically changes the behavior of the system when the
cluster's DNS is unavailable.

The ClusterFirstWithHostNet policy is lacking any amount of detail as to
why one would desire to use it. This commit doesn't go into complete
detail but adds an important note about the behavior of `hostNetwork`
when not using the `ClusterFirstWIthHostNet` Policy.

content/en/docs/concepts/services-networking/dns-pod-service.md

@@ -220,13 +220,15 @@ following Pod-specific DNS policies. These policies are specified in the
   See [related discussion](/docs/tasks/administer-cluster/dns-custom-nameservers)
   for more details.
 - "`ClusterFirst`": Any DNS query that does not match the configured cluster
-  domain suffix, such as "`www.kubernetes.io`", is forwarded to the upstream
-  nameserver inherited from the node. Cluster administrators may have extra
+  domain suffix, such as "`www.kubernetes.io`", is forwarded to an upstream
+  nameserver by the DNS server. Cluster administrators may have extra
   stub-domain and upstream DNS servers configured.
   See [related discussion](/docs/tasks/administer-cluster/dns-custom-nameservers)
   for details on how DNS queries are handled in those cases.
 - "`ClusterFirstWithHostNet`": For Pods running with hostNetwork, you should
-  explicitly set its DNS policy "`ClusterFirstWithHostNet`".
+  explicitly set its DNS policy to "`ClusterFirstWithHostNet`". Otherwise, Pods
+  running with hostNetwork and `"ClusterFirst"` will fallback to the behavior
+  of the `"Default"` policy.
   - Note: This is not supported on Windows. See [below](#dns-windows) for details
 - "`None`": It allows a Pod to ignore DNS settings from the Kubernetes
   environment. All DNS settings are supposed to be provided using the