fix golint issues in core/iox (#488)

core/iox/nopcloser.go

@@ -10,6 +10,7 @@ func (nopCloser) Close() error {
 	return nil
 }
 
+// NopCloser returns a io.WriteCloser that does nothing on calling Close.
 func NopCloser(w io.Writer) io.WriteCloser {
 	return nopCloser{w}
 }