cmd/go: mention GONOSUMDB wherever GONOPROXY is mentioned

[tbpg]

What version of Go are you using (go version)?

$ go version
go version devel +0f897f916a Tue May 28 02:52:39 2019 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/tbp/.cache/go-build"
GOENV="/home/tbp/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/tbp/go"
GOPROXY="https://proxy.golang.org"
GOROOT="/home/tbp/code/gotip"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/tbp/code/gotip/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build394386353=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version devel +0f897f916a Tue May 28 02:52:39 2019 +0000 linux/amd64
GOROOT/bin/go tool compile -V: compile version devel +0f897f916a Tue May 28 02:52:39 2019 +0000
uname -sr: Linux 4.19.28-2rodete1-amd64
Distributor ID:	Debian
Description:	Debian GNU/Linux rodete
Release:	rodete
Codename:	rodete
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Debian GLIBC 2.24-12) stable release version 2.24, by Roland McGrath et al.

What did you do?

Read cmd/go docs about GONOPROXY and didn't immediately find GONOSUMDB. Related to #32184 and #32056.

What did you expect to see?

GONOSUMDB documented wherever GONOPROXY is.

What did you see instead?

Hard to find docs about GONOSUMDB.

[gopherbot]

Change https://golang.org/cl/179219 mentions this issue: cmd/go: document GO{NO}SUMDB in 'go help environment' and 'go help modules'