doc/go1.4.txt: document that C sources in non-cgo packages are no longer allowed
doc/go1.4.txt: document that C sources in non-cgo packages are no longer allowed

CC=golang-codereviews
https://codereview.appspot.com/145890046

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