Add path, existingfile and existingdir types.

- Document custom types.
- Add docker example.

_examples/shell/main.go

@@ -24,9 +24,13 @@ var cli struct {
 
 func main() {
 	cmd := kong.Parse(&cli, kong.Description("A shell-like example app."),
-		kong.HelpOptions(kong.CompactHelp()))
+		kong.UsageOnError(),
+		kong.HelpOptions(kong.HelpPrinterOptions{
+			Compact: true,
+			Summary: true,
+		}))
 	switch cmd {
-	case "rm <paths>":
+	case "rm <path>":
 		fmt.Println(cli.Rm.Paths, cli.Rm.Force, cli.Rm.Recursive)
 
 	case "ls":