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

@@ -369,11 +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")