cmd/go: revise 'go help' documentation for modules

Module-related help pages now contain a brief summary and point to the
reference documentation at golang.org/ref/mod for details.

Help pages for commands like 'go get' still describe the basic usage
and summarize flags but don't provide as much background detail.

Fixes #41427
Fixes #43419

Change-Id: Icacd38e0f33c352c447cc5a496c99674493abde2
Reviewed-on: https://go-review.googlesource.com/c/go/+/282615


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