docs: add doc and example for CommandInfo.Opt method

Author: steverusso

builder.go

@@ -144,6 +144,9 @@ func (c CommandInfo) Usage(lines ...string) CommandInfo {
 	return c
 }
 
+// Opt adds o as an option to this CommandInfo. This method will panic if the option has
+// neither a long or short name set (this should never happen when using the builder
+// pattern starting with the [NewOpt] function or its siblings).
 func (c CommandInfo) Opt(o InputInfo) CommandInfo {
 	// Assert `o` is not a positional arg by making sure it has at least one option name.
 	if o.NameShort == "" && o.NameLong == "" {

examples_test.go

@@ -196,6 +196,16 @@ func ExampleCommandInfo_Arg() {
 	// alice
 }
 
+func ExampleCommandInfo_Opt() {
+	c := cli.New().
+		Opt(cli.NewOpt("a")).
+		ParseOrExit("-a", "hello")
+
+	fmt.Println(cli.Get[string](c, "a"))
+	// Output:
+	// hello
+}
+
 func ExampleCommandInfo_ExtraHelp() {
 	in := cli.New("example").
 		Help("an example command").