cmd/go: use current go source code when generating doc.go

Change-Id: Iad1764707d173a09467fd36e8c49a58147f12219
Reviewed-on: https://go-review.googlesource.com/4320
Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/go/doc.go

@@ -310,6 +310,7 @@ The generator is run in the package's source directory.
 Go generate accepts one specific flag:
 
 	-run=""
+		TODO: This flag is unimplemented.
 		if non-empty, specifies a regular expression to
 		select directives whose command matches the expression.
 

src/cmd/go/mkdoc.sh

@@ -1,9 +1,12 @@
-#!/bin/sh
+#!/bin/bash
 # Copyright 2012 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.
 
-go install # So the next line will produce updated documentation.
-go help documentation | sed 's; \*/; * /;' >doc.go
-gofmt -w doc.go
+set -e
+
+go build -o go.latest
+./go.latest help documentation | sed 's; \*/; * /;' >doc.go
+gofmt -w doc.go
+rm go.latest