cgo: note that CString result must be freed

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4968054

src/cmd/cgo/doc.go

@@ -72,7 +72,7 @@ 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
+	// Go string to C string - result is not garbage collected
 	func C.CString(string) *C.char
 
 	// C string to Go string

src/pkg/runtime/cgo/util.c

@@ -18,7 +18,7 @@ x_cgo_malloc(void *p)
 
 void (*_cgo_malloc)(void*) = x_cgo_malloc;
 
-/* Stub for calling from Go */
+/* Stub for calling free from Go */
 static void
 x_cgo_free(void *p)
 {