diff --git a/src/io/io.go b/src/io/io.go
index 2010770e6a..9cc3086c19 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -368,6 +368,9 @@ func Copy(dst Writer, src Reader) (written int64, err error) {
 // provided buffer (if one is required) rather than allocating a
 // temporary one. If buf is nil, one is allocated; otherwise if it has
 // zero length, CopyBuffer panics.
+//
+// If either src implements WriterTo or 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")