encoding/json: add an example for InputOffset() function

[Windsooon]

What version of Go are you using (go version)?

$ go go1.16.4 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What did you expect to see?

Add an example for it.

[Windsooon]

We can reuse the example from Decoder, the new example looks like:

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"strings"
)

func main() {
	const jsonStream = `
	{"Name": "Ed", "Text": "Knock knock."}
	{"Name": "Sam", "Text": "Who's there?"}
	{"Name": "Ed", "Text": "Go fmt."}
	{"Name": "Sam", "Text": "Go fmt who?"}
	{"Name": "Ed", "Text": "Go fmt yourself!"}
`
	type Message struct {
		Name, Text string
	}
	dec := json.NewDecoder(strings.NewReader(jsonStream))
	var m Message
	err := dec.Decode(&m);
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%s: %s\n", m.Name, m.Text)
        // print the input offset
	fmt.Println(dec.InputOffset())
}

[ameowlia]

Were you planning to submit a fix @Windsooon ? If not, I can do this.

[dsnet]

Let's hold off on adding an example, until we resolve #42571.