cmd/cgo: document the cgo types also follow Go name space rules. Fixes issue 7958.
cmd/cgo: document the cgo types also follow Go name space rules.
Fixes issue 7958.

LGTM=rsc
R=golang-codereviews, rsc, r, gobot
CC=golang-codereviews
https://codereview.appspot.com/91520043

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