Display multiple optional args in a more expected way.

Previously:

    Usage: command [<one>] [<two>]

Now:

    Usage: command [<one> [<two>]]

Thanks to @juliaogris for the inspiration!

help_test.go

@@ -21,6 +21,38 @@ func (threeArg) Help() string {
 	return `Detailed help provided through the HelpProvider interface.`
 }
 
+func TestHelpOptionalArgs(t *testing.T) {
+	var cli struct {
+		One string `arg:"" optional:"" help:"One optional arg."`
+		Two string `arg:"" optional:"" help:"Two optional arg."`
+	}
+	w := bytes.NewBuffer(nil)
+	exited := false
+	app := mustNew(t, &cli,
+		kong.Name("test-app"),
+		kong.Writers(w, w),
+		kong.Exit(func(int) {
+			exited = true
+			panic(true) // Panic to fake "exit".
+		}),
+	)
+	require.PanicsWithValue(t, true, func() {
+		_, err := app.Parse([]string{"--help"})
+		require.NoError(t, err)
+	})
+	require.True(t, exited)
+	expected := `Usage: test-app [<one> [<two>]]
+
+Arguments:
+  [<one>]    One optional arg.
+  [<two>]    Two optional arg.
+
+Flags:
+  -h, --help    Show context-sensitive help.
+`
+	require.Equal(t, expected, w.String())
+}
+
 func TestHelp(t *testing.T) {
 	// nolint: govet
 	var cli struct {

model.go

@@ -146,11 +146,17 @@ func (n *Node) Summary() string {
 		summary += " " + flags
 	}
 	args := []string{}
+	optional := 0
 	for _, arg := range n.Positional {
-		args = append(args, arg.Summary())
+		summary := arg.Summary()
+		if arg.Tag.Optional {
+			optional++
+			summary = strings.TrimRight(summary, "]")
+		}
+		args = append(args, summary)
 	}
 	if len(args) != 0 {
-		summary += " " + strings.Join(args, " ")
+		summary += " " + strings.Join(args, " ") + strings.Repeat("]", optional)
 	} else if len(n.Children) > 0 {
 		summary += " <command>"
 	}