Modify kubectl exec help to set -c CONTAINER as optional

pkg/kubectl/cmd/exec.go

@@ -53,7 +53,7 @@ func NewCmdExec(f *cmdutil.Factory, cmdIn io.Reader, cmdOut, cmdErr io.Writer) *
 		Executor: &DefaultRemoteExecutor{},
 	}
 	cmd := &cobra.Command{
-		Use:     "exec POD -c CONTAINER -- COMMAND [args...]",
+		Use:     "exec POD [-c CONTAINER] -- COMMAND [args...]",
 		Short:   "Execute a command in a container.",
 		Long:    "Execute a command in a container.",
 		Example: exec_example,
@@ -65,7 +65,7 @@ func NewCmdExec(f *cmdutil.Factory, cmdIn io.Reader, cmdOut, cmdErr io.Writer) *
 	}
 	cmd.Flags().StringVarP(&options.PodName, "pod", "p", "", "Pod name")
 	// TODO support UID
-	cmd.Flags().StringVarP(&options.ContainerName, "container", "c", "", "Container name")
+	cmd.Flags().StringVarP(&options.ContainerName, "container", "c", "", "Container name. If omitted, the first container in the pod will be chosen")
 	cmd.Flags().BoolVarP(&options.Stdin, "stdin", "i", false, "Pass stdin to the container")
 	cmd.Flags().BoolVarP(&options.TTY, "tty", "t", false, "Stdin is a TTY")
 	return cmd