cgo: explain how to free something

R=golang-dev, n13m3y3r, r
CC=golang-dev
https://golang.org/cl/4958053

src/cmd/cgo/doc.go

@@ -72,7 +72,10 @@ the pointer to the first element explicitly: C.f(&x[0]).
 A few special functions convert between Go and C types
 by making copies of the data.  In pseudo-Go definitions:
 
-	// Go string to C string - result is not garbage collected
+	// Go string to C string
+	// The C string is allocated in the C heap using malloc.
+	// It is the caller's responsibility to arrange for it to be
+	// freed, such as by calling C.free.
 	func C.CString(string) *C.char
 
 	// C string to Go string