go/build: update docs for GOOS.go change
go/build: update docs for GOOS.go change

Forgotten in https://codereview.appspot.com/147690043/

Update Issue 8838

LGTM=r
R=r
CC=golang-codereviews, rsc
https://codereview.appspot.com/152220045

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go