$ go tool -h says:

  For more about each tool command, see 'go tool command -h'.

but it was suggested to change the suggestion to say:

  see 'go doc command'

In #18313.

That would work for every tool except dist, which has no doc.go.

This change adds a doc.go file to cmd/dist.

Updates #18313

Change-Id: If67a21934b87647a69359d9c14d8de3775c587b7
Reviewed-on: https://go-review.googlesource.com/54351
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>