Write about positional fmt arguments

[colega]

See example:

package main

import (
	"fmt"
)

func main() {
	fmt.Printf("%v, %[1]v, %v, %[1]v, %v\n", 1, 2, 3, 4, 5)
	fmt.Printf("%v, %[1]v, %v, %[3]v, %v\n", 1, 2, 3, 4, 5)
	fmt.Printf("%v, %[5]v, %v, %[3]v, %v\n", 1, 2, 3, 4, 5)
	fmt.Printf("%v, %[1]v, %v, %[2]v, %v\n", 1, 2, 3, 4, 5)
}