cmd/go: improve module version query documentation

Add "upgrade" and "patch" to 'go help modules' section 'Module queries'.
Also explicitly call out the fact that @v2 will select the latest
version starting with v2, not the branch named v2, since this is a
common source of confusion.

Fixes #33010

Change-Id: I2fe27543b81a160fb6f6b8e8444a7a35f3661433
Reviewed-on: https://go-review.googlesource.com/c/go/+/185979


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