docs: mark six deferred review findings with TODOs

Behavior untouched; each marker records a known gap and the intended
fix so the next visit doesn't rediscover it: transient zero-sent
sendmmsg errors drop a whole run; the non-vnet Poll queue lacks the
post-wake drain loop; recvmmsg controllen resets touch every entry;
cached handshake packets flush one syscall each; the routines clamp in
activate() would blackhole surplus REUSEPORT sockets if it ever became
reachable; darwin WriteBatch burst-drops on EWOULDBLOCK.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ugV2edVqoz3tBvq9J6yWp

udp/udp_linux.go

@@ -313,6 +313,9 @@ func (u *StdConn) ListenOut(r EncReader, flush func()) error {
 
 	for {
 		if cmsgSpace > 0 {
+			// TODO: the kernel only rewrites Controllen on entries it fills,
+			// so resetting just the first `n` from the previous wakeup would
+			// save ~(batch-n) stores per wakeup on trickle traffic.
 			for i := range msgs {
 				setMsgControllen(&msgs[i].Hdr, cmsgSpace)
 			}