cmd/go: add basic docs on calling between Go and C/C++

This is a framework for docs on the subject more than it is
actual docs.

The section header in go/doc.go just says "C", not "C/C++,"
because otherwise godoc doesn't recognize the line as a
section header.

Fixes issue 5473.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13280050
cmd/gc: fix set but not used error

R=rsc, r
CC=golang-dev
https://codereview.appspot.com/13749044