tcpip.Endpoint.Write() doc is out of date

The doc says
```go
// Unlike io.Writer.Write, Endpoint.Write transfers ownership of any bytes
// successfully written to the Endpoint. That is, if a call to
// Write(SlicePayload{data}) returns (n, err), it may retain data[:n], and
// the caller should not use data[:n] after Write returns.
```
but the signature of `Write` has since changed. It no longer accepts a slice payload, so this advice is no longer applicable.