diff --git a/src/io/io.go b/src/io/io.go
index a65f33ba6e..7c3539ac88 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -369,11 +369,8 @@ func Copy(dst Writer, src Reader) (written int64, err error) {
 // temporary one. If buf is nil, one is allocated; otherwise if it has
 // zero length, CopyBuffer panics.
 //
-// If src implements the WriterTo interface,
-// the copy is implemented by calling src.WriteTo(dst).
-// Otherwise, if dst implements the ReaderFrom interface,
-// the copy is implemented by calling dst.ReadFrom(src). In both cases,
-// buf will be ignored and will not be used.
+// If either src implements WriterTo or if dst implements ReaderFrom,
+// buf will not be used to perform the copy.
 func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 	if buf != nil && len(buf) == 0 {
 		panic("empty buffer in io.CopyBuffer")