testing: add available godoc link

testing: add available godoc link

Change-Id: I8f4d097601796f53176d490cddf8832b7caa4c05
Reviewed-on: https://go-review.googlesource.com/c/go/+/539836
Run-TryBot: shuang cui <imcusg@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>