Improve portforward command's description which hint the requirment of 'socat' command

pkg/kubectl/cmd/portforward/portforward.go

@@ -59,7 +59,7 @@ type PortForwardOptions struct {
 
 var (
 	portforwardLong = templates.LongDesc(i18n.T(`
-                Forward one or more local ports to a pod.
+                Forward one or more local ports to a pod. This command requires the node to have 'socat' installed.
 
                 Use resource type/name such as deployment/mydeployment to select a pod. Resource type defaults to 'pod' if omitted.