Since at least 1.0.3, the testing package has said that logs are dumped
to standard error, but has in fact dumped the logs to standard output.
We could change to dump to standard error, but after doing it this way
for so long I think it's better to change the docs.

Fixes #16138.

Change-Id: If39c7ce91f51c7113f33ebabfb8f84fd4611b9e1
Reviewed-on: https://go-review.googlesource.com/24311
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>