testing: document that benchmark logs are dumped to stdout

The doc for testing.T and testing.B said that both test and benchmark
logs are printed to stderr, but in reality that wasn't the case.

CL 24311 fixed the doc for T, this change fixes it for B.

Fixes #36257

Change-Id: I0ff77ff44608f60320a1565b371c81e96039e71c
Reviewed-on: https://go-review.googlesource.com/c/go/+/212457


Reviewed-by: Gabriel Aszalos <gabriel.aszalos@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Gabriel Aszalos <gabriel.aszalos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>