io: CopyBuffer should document when buf is not used

[sandyskies]

The follow is the comment for io.CopyBuffer,
// CopyBuffer is identical to Copy except that it stages through the
// 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.
func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error)

which is not precise enough, in the case of src implements the WriterTo interface or the case of if dst implements the ReaderFrom interface , buf will be ignore ,which is not mentioned at all. Which is very easy to lead people to wrong way.

[bcmills]

This closely relates to #16474.

[sandyskies]

@bcmills Before the API is changed, we should add more comment to explain the behavior. This PR
is for adding the comment. #32279

[gopherbot]

Change https://golang.org/cl/179137 mentions this issue: io: make CopyBuffer docs state when buf is not used