cmd/go: update "go help doc" docs

cmd/go: update "go help doc" docs

This CL updates "go help doc" docs so they reflect the following
changes:

- CL 59413 modified "go doc", so the behavior of the two-args case is
  consistent with the one-arg one.
- CL 141397 removed godoc's command-line interface in favor of "go doc".

Fixes #49830.

Change-Id: I0923634291d34ae663fe2944d69757462b814919
Reviewed-on: https://go-review.googlesource.com/c/go/+/367497
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Russ Cox <rsc@golang.org>