cmd/dist: add doc file to fix go doc dist
$ 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>