bufio: clarify that Flush returns a cached write error

Change-Id: I377403fc0981d58aec5d84a1dd0d4e08532a575c
Reviewed-on: https://go-review.googlesource.com/41291Reviewed-by: Dan Peterson <dpiddy@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bufio/bufio.go

@@ -513,7 +513,7 @@ func (b *Reader) writeBuf(w io.Writer) (int64, error) {
 
 // Writer implements buffering for an io.Writer object.
 // If an error occurs writing to a Writer, no more data will be
-// accepted and all subsequent writes will return the error.
+// accepted and all subsequent writes, and Flush, will return the error.
 // After all data has been written, the client should call the
 // Flush method to guarantee all data has been forwarded to
 // the underlying io.Writer.