io: make CopyBuffer docs state when buf is not used

Document that if either src implements the WriteTo interface
or if dst implements the ReaderFrom interface, then
buf will not be used.

Fixes #32276

Author: sandyskies

src/io/io.go

@@ -369,7 +369,7 @@ 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 either src implements WriterTo or if dst implements ReaderFrom,
+// 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 {