cmd/go: regenerate doc.go

Move change from CL 170770043 to correct file and regenerate docs
for changes from CL 164120043.

LGTM=adg
R=golang-codereviews, adg, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/183000043

Committer: Andrew Gerrand <adg@golang.org>