add some specifications on the top command

pkg/kubectl/cmd/top.go

@@ -32,13 +32,15 @@ var (
 	topLong = templates.LongDesc(`
 		Display Resource (CPU/Memory/Storage) usage.
 
-		The top command allows you to see the resource consumption for nodes or pods.`)
+		The top command allows you to see the resource consumption for nodes or pods.
+
+		This command is available only if the Heapster works fine on the server. `)
 )
 
 func NewCmdTop(f cmdutil.Factory, out, errOut io.Writer) *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "top",
-		Short: "Display Resource (CPU/Memory/Storage) usage",
+		Short: "Display Resource (CPU/Memory/Storage) usage, available only if Heapster works fine on the server.",
 		Long:  topLong,
 		Run:   cmdutil.DefaultSubCommandRun(errOut),
 	}