multiport: bring docs and comments in line with the lane design

examples/config.yml still described lanes as extra tunnels with their own
handshakes, built lazily per routine. Rewrite it for lanes as sessions derived
from the base tunnel, and say that multiport.lanes only bounds sending.

In design/multiport.md, correct the partner-lane pairing (our lane s pairs
with the peer's lane s+portOffset, which laneBias lines up), the asymmetric
host example, the retry backoff, which reader owns what, the full-path
diagram, the demand-after-reset story and the metric descriptions.

Drop comments left over from earlier iterations: base traffic on "socket 0",
lanes picked by routine, lane handshakes, and multiport defaulting on.

inside.go

@@ -156,14 +156,14 @@ func (f *Interface) sendInsideEncrypt(hostinfo *HostInfo, ci *ConnectionState, l
 //
 // When this flow has a usable multiport lane to this peer, the direct path swaps
 // to that lane's session and socket below. Relay and base traffic stays on
-// tx.base (socket 0).
+// tx.base, this routine's socket on the base port.
 func (f *Interface) sendInsideMessage(hostinfo *HostInfo, pkt tio.Packet, fwPacket *firewall.Packet, nb []byte, tx *txQueue) {
 	ci := hostinfo.ConnectionState
 	if ci.eKey == nil {
 		return
 	}
 
-	// Base and relay traffic stays on socket 0; the direct path may swap to tx.lane below.
+	// Base and relay traffic stays on the base port; the direct path may swap to tx.lane below.
 	sendBatch := tx.base
 
 	// One traffic-out mark covers every segment of the superpacket; doing it