encoding/json: MarshalIndent does not prefix first line

[cloneable]

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

$ go version
go version go1.17.2 darwin/amd64

Does this issue reproduce with the latest release?

Yes, also gotip.

What did you do?

https://play.golang.org/p/jSVnZBwYiwr

x := map[string]interface{}{
	"field": "value",
}
data, err := json.MarshalIndent(x, "*** ", "  ")
if err != nil {
	log.Fatal(err)
}
fmt.Println(string(data))

What did you expect to see?

*** {
***   "field": "value"
*** }

What did you see instead?

{
***   "field": "value"
*** }

[cloneable]

Never mind. Looking at encoding_test.go it seems this is intentional to support indentation in source code. The description for MarshalIdent could be made clearer.