run.bash, cmd/dist: document GO_TEST_SHORT and GO_TEST_TIMEOUT_SCALE
These environment variables affect cmd/dist, and in turn run.bash.
They exist primarily for the Go build system, but are still needed
sometimes when investigating problems. Document them in one place.
Fixes #46054.
Change-Id: I5dea2ac79b0d203d2f3c9ec2980382f62bead5cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/455517
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>