cmd/go: clarify documentation for 'go get -u'

Also: add a comment in internal/get.newUpgrader as a follow-up to
CL 177677.

Updates #26902

Change-Id: Ibce2807ecb44fa21697ca04a51c44ddca0f661d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/176902


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