cmd/cgo: document CGO_LDFLAGS and CGO_CFLAGS
cmd/cgo: document CGO_LDFLAGS and CGO_CFLAGS

Fixes issue 3203.

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/5769043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang