[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
b816cb2cd5f1b7cee660c6e4b85d8130b960752a)
Reviewed-on: https://go-review.googlesource.com/c/go/+/224119
TryBot-Result: Gobot Gobot <gobot@golang.org>