Prior to this change "go tool vet -all -shadow" ran only -shadow check.

Also fix godoc package path in the usage text.

Fixes #13020

Change-Id: I87c60d6b06a02106ae8bff56adb79df032cc4646
Reviewed-on: https://go-review.googlesource.com/16325Reviewed-by: Russ Cox <rsc@golang.org>