testing: update docs for fuzzcachedir

Although most of the code seems to be already implemented
to support this for general use, it didn't make it in for
Go 1.18, so for now we should at least document that it's
only for use by the go command.

Change-Id: Id559e72d590aedeaaa50bcf880bca1a385d858dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/370954


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>