docs(flag): add UseShortOptionHandling description

Signed-off-by: Kevin Cui <bh@bugs.cc>

docs/v2/examples/flags.md

@@ -104,9 +104,11 @@ See full list of flags at https://pkg.go.dev/github.com/urfave/cli/v2
 
 For bool flags you can specify the flag multiple times to get a count(e.g -v -v -v or -vvv)
 
+> If you want to support the `-vvv` flag, you need to set `App.UseShortOptionHandling`.
+
 <!-- {
-  "args": ["&#45;&#45;foo", "&#45;&#45;foo"],
-  "output": "count 2"
+  "args": ["&#45;&#45;foo", "&#45;&#45;foo", "&#45;fff",  "&#45;f"],
+  "output": "count 6"
 } -->
 ```go
 package main
@@ -123,10 +125,12 @@ func main() {
 	var count int
 
 	app := &cli.App{
+		UseShortOptionHandling: true,
 		Flags: []cli.Flag{
 			&cli.BoolFlag{
 				Name:        "foo",
 				Usage:       "foo greeting",
+				Aliases:     []string{"f"},
 				Count: &count,
 			},
 		},