doc: explain how to use "go vet -shadow"
doc: explain how to use "go vet -shadow"

Fixes #29260

Change-Id: I419b74d06380113f4bd32b9aeb053c3be36208d5
Reviewed-on: https://go-review.googlesource.com/c/154584
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

cloudflare/go,CAFxX/go,akutz/go,nawawi/go,golang/go,golang/go,akutz/go,CAFxX/go,akutz/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,akutz/go,nawawi/go,nawawi/go,josharian/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,golang/go,josharian/go,CAFxX/go,akutz/go,CAFxX/go,josharian/go,cloudflare/go,josharian/go,nawawi/go,josharian/go,josharian/go,cloudflare/go,josharian/go,CAFxX/go,nawawi/go,golang/go,golang/go,cloudflare/go,nawawi/go