Make arguments semantics clear without the need to look for
json.Indent documentation.

Change-Id: If9adfe9f477a30d426ae83790b0f2578c0a809b7
Reviewed-on: https://go-review.googlesource.com/61670
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>