encoding/json: add example for RawMessage marshalling

Fixes #16648

Change-Id: I3ab21ab33ca3f41219de9518ac6a39f49131e5e5
Reviewed-on: https://go-review.googlesource.com/26692
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/example_test.go

@@ -174,7 +174,7 @@ func ExampleDecoder_Decode_stream() {
 }
 
 // This example uses RawMessage to delay parsing part of a JSON message.
-func ExampleRawMessage() {
+func ExampleRawMessage_unmarshal() {
 	type Color struct {
 		Space string
 		Point json.RawMessage // delay parsing until we know the color space
@@ -219,6 +219,30 @@ func ExampleRawMessage() {
 	// RGB &{98 218 255}
 }
 
+// This example uses RawMessage to use a precomputed JSON during marshal.
+func ExampleRawMessage_marshal() {
+	h := json.RawMessage(`{"precomputed": true}`)
+
+	c := struct {
+		Header *json.RawMessage `json:"header"`
+		Body   string           `json:"body"`
+	}{Header: &h, Body: "Hello Gophers!"}
+
+	b, err := json.MarshalIndent(&c, "", "\t")
+	if err != nil {
+		fmt.Println("error:", err)
+	}
+	os.Stdout.Write(b)
+
+	// Output:
+	// {
+	// 	"header": {
+	// 		"precomputed": true
+	// 	},
+	// 	"body": "Hello Gophers!"
+	// }
+}
+
 func ExampleIndent() {
 	type Road struct {
 		Name   string