[release-branch.go1.14] cmd/go: update 'go help modules' for automatic vendoring

* Mention vendor/modules.txt verification with -mod=vendor.
* Update "Modules and vendoring" section.

Fixes #37931

Change-Id: Ia3ee72457daabaa2fc15b7ea7427eb324c088b8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/223926


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit b816cb2c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/224119
TryBot-Result: Gobot Gobot <gobot@golang.org>