docs: align comments with the code they describe

- multi_coalesce/batch: the ordering contract now states what Flush
  actually guarantees -- per-flow DATA order -- and names the two
  shapes later data may legally overtake (pure ACKs by design, and
  unparseable in-flow shapes as an accepted tradeoff).
- validVnetHdr claimed DATA_VALID makes the stack skip L4 checksum
  verification; the tun write path ignores that bit entirely. What the
  header buys is the absence of NEEDS_CSUM.
- tun_darwin Write said "only valid for single threaded use"; it is
  concurrency-safe and concurrent callers exist.
- udp_coalesce eviction comment said "Seal it" but never sets sealed.
- recordCapability: note the gauges are process-global while the state
  is per-socket (last writer wins).
- drop a stale tunReadBufSize reference.

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

overlay/batch/udp_coalesce.go

@@ -160,7 +160,8 @@ func (c *UDPCoalescer) commitParsed(pkt []byte, info parsedUDP) error {
 			}
 			return nil
 		}
-		// Can't extend. Seal it and fall through to seed a fresh slot.
+		// Can't extend: evict it from openSlots and fall through to seed a
+		// fresh slot. (Eviction only; sealed is never set here.)
 		delete(c.openSlots, info.fk)
 		if c.lastSlot == open {
 			c.lastSlot = nil