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

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