The comment for Cmd.Stdout and Cmd.Stderr says that it's safe to
set both to the same writer, but it doesn't say that this only
works when both writers are comparable.

This change updates the comment to explain that using a
non-comparable writer may still lead to a race.

Fixes #19804

Change-Id: I63b420034666209a2b6fab48b9047c9d07b825e2
Reviewed-on: https://go-review.googlesource.com/42052Reviewed-by: Ian Lance Taylor <iant@golang.org>