compress/flate: clarify the behavior of Writer.Flush

Fixes golang#16068

Change-Id: I04e80a181c0b7356996f7a1158ea4895ff9e1e39
Reviewed-on: https://go-review.googlesource.com/28477
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/flate/deflate.go

@@ -702,10 +702,12 @@ func (w *Writer) Write(data []byte) (n int, err error) {
 	return w.d.write(data)
 }
 
-// Flush flushes any pending compressed data to the underlying writer.
+// Flush flushes any pending data to the underlying writer.
 // It is useful mainly in compressed network protocols, to ensure that
 // a remote reader has enough data to reconstruct a packet.
 // Flush does not return until the data has been written.
+// Calling Flush when there is no pending data still causes the Writer
+// to emit a sync marker of at least 4 bytes.
 // If the underlying writer returns an error, Flush returns that error.
 //
 // In the terminology of the zlib library, Flush is equivalent to Z_SYNC_FLUSH.