encoding/json: add example for Indent, clarify the docs.

There was confusion in the behavior of json.Indent; This change
attempts to clarify the behavior by providing a bit more verbiage
to the documentation as well as provide an example function.

Fixes #7821.

LGTM=robert.hencke, adg
R=golang-codereviews, minux.ma, bradfitz, aram, robert.hencke, r, adg
CC=golang-codereviews
https://golang.org/cl/97840044

src/pkg/encoding/json/example_test.go

@@ -5,6 +5,7 @@
 package json_test
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -127,3 +128,34 @@ func ExampleRawMessage() {
 	// YCbCr &{255 0 -10}
 	// RGB &{98 218 255}
 }
+
+func ExampleIndent() {
+	type Road struct {
+		Name   string
+		Number int
+	}
+	roads := []Road{
+		{"Diamond Fork", 29},
+		{"Sheep Creek", 51},
+	}
+
+	b, err := json.Marshal(roads)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var out bytes.Buffer
+	json.Indent(&out, b, "=", "\t")
+	out.WriteTo(os.Stdout)
+	// Output:
+	// [
+	// =	{
+	// =		"Name": "Diamond Fork",
+	// =		"Number": 29
+	// =	},
+	// =	{
+	// =		"Name": "Sheep Creek",
+	// =		"Number": 51
+	// =	}
+	// =]
+}