cgo: document export of Go functions to C.
cgo: document export of Go functions to C.

R=mpimenov, rsc
CC=golang-dev, remy
http://codereview.appspot.com/5376082

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go