String method comments should explain what they do,
not that they are attempting to implement fmt.Stringer.

Change-Id: If51dd1ff2f0c2f9ef9dca569bfa0c3914be2e8fe
Reviewed-on: https://go-review.googlesource.com/82081
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>