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.

control.go

@@ -212,7 +212,7 @@ func (c *Control) RebindUDPServer() {
 		return
 	}
 
-	// Every socket needs rebinding, not just the base: with multiport each one is bound to its own lane port, and
+	// Every socket needs rebinding, not just the base: with multiport the sockets are spread over the lane ports, and
 	// even without it the surplus SO_REUSEPORT sockets stay pinned to the interface we came up on otherwise.
 	//
 	// A failure here means we are likely still pinned to the interface we came up on, so the rest of this is
@@ -420,7 +420,7 @@ func copyHostInfo(h *HostInfo, preferredRanges []netip.Prefix) ControlHostInfo {
 
 // copyLanes snapshots the sendable multiport lanes of a tunnel, or nil when it
 // has none. txAddr is the lane's gate as well as its destination, so a nil load
-// is exactly "this lane is down and its routine is riding the base tunnel".
+// is exactly "this lane is down and its flows are riding the base tunnel".
 func copyLanes(h *HostInfo) []ControlLane {
 	ls := h.lanes
 	if ls == nil || ls.txLanes < 2 {