io/ioutil: add a comment on why devNull is a ReaderFrom

... protects this optimization from future well-meaning
Gophers looking to delete unused code. :)

R=gri
CC=golang-dev
https://golang.org/cl/5165041

src/pkg/io/ioutil/ioutil.go

@@ -104,6 +104,10 @@ func NopCloser(r io.Reader) io.ReadCloser {
 
 type devNull int
 
+// devNull implements ReaderFrom as an optimization so io.Copy to
+// ioutil.Discard can avoid doing unnecessary work.
+var _ io.ReaderFrom = devNull(0)
+
 func (devNull) Write(p []byte) (int, os.Error) {
 	return len(p), nil
 }