net: document why we do not use SO_REUSEADDR on windows

R=rsc, adg
CC=golang-dev
https://golang.org/cl/5302058

src/pkg/net/sock_windows.go

@@ -11,6 +11,13 @@ import (
 )
 
 func setKernelSpecificSockopt(s syscall.Handle, f int) {
+	// Windows will reuse recently-used addresses by default.
+	// SO_REUSEADDR should not be used here, as it allows
+	// a socket to forcibly bind to a port in use by another socket.
+	// This could lead to a non-deterministic behavior, where
+	// connection requests over the port cannot be guaranteed
+	// to be handled by the correct socket.
+
 	// Allow broadcast.
 	syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_BROADCAST, 1)