diff --git a/src/pkg/encoding/xml/xml.go b/src/pkg/encoding/xml/xml.go
index 5b9d670024..b473cb8458 100644
--- a/src/pkg/encoding/xml/xml.go
+++ b/src/pkg/encoding/xml/xml.go
@@ -200,6 +200,8 @@ type Decoder struct {
 }
 
 // NewDecoder creates a new XML parser reading from r.
+// If r does not implement io.ByteReader, NewDecoder will
+// do its own buffering.
 func NewDecoder(r io.Reader) *Decoder {
 	d := &Decoder{
 		ns:       make(map[string]string),