go.tools/misc/dist: copy doc.go from go.tools to go root
go.tools/misc/dist: copy doc.go from go.tools to go root

This will allow "godoc godoc", "godoc vet", "godoc cover" to work.

Fixes issue 6527.

R=r, dsymonds
CC=golang-dev
https://codereview.appspot.com/14566049

firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang