CL 21057 added this method during the Go 1.7 cycle
(so it is not yet released and still possible to revise).

This makes it clearer that the method is not doing something
(like func Indent does), but just changing a setting about doing
something later.

Also document that this is in some sense irreversible.
I think that's probably a mistake but the original CL discussion
claimed it as a feature, so I'll leave it alone.

For #6492.

Change-Id: If4415c869a9196501056c143811a308822d5a420
Reviewed-on: https://go-review.googlesource.com/23295Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>