cmd/go: go test flag -failfast should be cacheable

Add failfast to cacheable list and update docs

Fixes #47355

Change-Id: I75b371c45b80a3b179ff070b7b9d092a504380c0
GitHub-Last-Rev: abe61fd48c01fab4ef5ea5db013dcce4ead09c6f
GitHub-Pull-Request: golang/go#47371
Reviewed-on: https://go-review.googlesource.com/c/go/+/337229


Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>