cmd/go: document GOPROXY

[ainar-g]

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

Does this issue reproduce with the latest release?

go version devel +c80897b Mon Jul 16 20:29:54 2018 +0000 linux/amd64

What did you do?

$ go help environment | grep GOPROXY
$ go help modules | grep GOPROXY
$ go help mod | grep GOPROXY
$ go help module-get | grep GOPROXY

What did you expect to see?

The documentation for GOPROXY.

What did you see instead?

Nothing.

[FiloSottile]

/cc @rsc

[gopherbot]

Change https://golang.org/cl/124707 mentions this issue: cmd/go: document $GOPROXY, other module adjustments