update kubelet cluster-dns doc

cmd/kubelet/app/options/options.go

@@ -194,7 +194,7 @@ func (s *KubeletServer) AddFlags(fs *pflag.FlagSet) {
 	fs.BoolVar(&s.RegisterNode, "register-node", s.RegisterNode, "Register the node with the apiserver (defaults to true if --api-servers is set)")
 	fs.StringVar(&s.ClusterDomain, "cluster-domain", s.ClusterDomain, "Domain for this cluster.  If set, kubelet will configure all containers to search this domain in addition to the host's search domains")
 	fs.StringVar(&s.MasterServiceNamespace, "master-service-namespace", s.MasterServiceNamespace, "The namespace from which the kubernetes master services should be injected into pods")
-	fs.StringVar(&s.ClusterDNS, "cluster-dns", s.ClusterDNS, "IP address for a cluster DNS server.  If set, kubelet will configure all containers to use this for DNS resolution in addition to the host's DNS servers")
+	fs.StringVar(&s.ClusterDNS, "cluster-dns", s.ClusterDNS, "IP address for a cluster DNS server.  This value is used for containers' DNS server in case of Pods with \"dnsPolicy=ClusterFirst\"")
 	fs.DurationVar(&s.StreamingConnectionIdleTimeout.Duration, "streaming-connection-idle-timeout", s.StreamingConnectionIdleTimeout.Duration, "Maximum time a streaming connection can be idle before the connection is automatically closed. 0 indicates no timeout. Example: '5m'")
 	fs.DurationVar(&s.NodeStatusUpdateFrequency.Duration, "node-status-update-frequency", s.NodeStatusUpdateFrequency.Duration, "Specifies how often kubelet posts node status to master. Note: be cautious when changing the constant, it must work with nodeMonitorGracePeriod in nodecontroller. Default: 10s")
 	bindableNodeLabels := config.ConfigurationMap(s.NodeLabels)

docs/admin/kubelet.md

@@ -73,7 +73,7 @@ kubelet
       --chaos-chance=0: If > 0.0, introduce random client errors and latency. Intended for testing. [default=0.0]
       --cloud-config="": The path to the cloud provider configuration file.  Empty string for no configuration file.
       --cloud-provider="": The provider for cloud services.  Empty string for no provider.
-      --cluster-dns="": IP address for a cluster DNS server.  If set, kubelet will configure all containers to use this for DNS resolution in addition to the host's DNS servers
+      --cluster-dns="": IP address for a cluster DNS server.  This value is used for containers' DNS server in case of Pods with "dnsPolicy=ClusterFirst"
       --cluster-domain="": Domain for this cluster.  If set, kubelet will configure all containers to search this domain in addition to the host's search domains
       --config="": Path to the config file or directory of files
       --configure-cbr0[=false]: If true, kubelet will configure cbr0 based on Node.Spec.PodCIDR.