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>

src/cmd/dist/doc.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// dist is the bootstrapping tool for the Go distribution.
+//
+// Usage:
+//   go tool dist [command]
+//
+// The commands are:
+//   banner         print installation banner
+//   bootstrap      rebuild everything
+//   clean          deletes all built files
+//   env [-p]       print environment (-p: include $PATH)
+//   install [dir]  install individual directory
+//   list [-json]   list all supported platforms
+//   test [-h]      run Go test(s)
+//   version        print Go version
+package main