doc/go1.14: go command behavior with GO111MODULE=on outside modules

doc/go1.14: go command behavior with GO111MODULE=on outside modules

Hopefully this won't affect many people, but it's a substantial change
in behavior for a specific case, so it's probably worth mentioning.

Updates #32027

Change-Id: I61cd50e93d0199e94b011cc90150e072b6bb10ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/216597
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>