docs: state the at-least-once cost of the drain

A second Greptile pass flagged that a drained sibling's effect row
stays pending and later redelivers, a duplicate outbound request. The
duplicate is the documented at-least-once design: completing the
sibling would take over another worker's claim, skipping it would
break ordering, and the JS runtime behaves the same, so the docs now
state the trade explicitly instead of leaving it implied. The
concurrency race test already pins the behavior: order holds,
replays apply nothing, and every effect completes.

Author: cardmagic

docs/transmission.md

@@ -60,6 +60,15 @@ delivers every undelivered sibling for its actor up to its own mailbox
 sequence, oldest first. The receiving side dedups on `transmit:<effectId>`,
 so a redelivered envelope applies once.
 
+Delivery is at-least-once by design, and the drain accepts redundant sends
+as the price of ordering without cross-worker coordination. A drained
+sibling's own effect row stays pending, because completing it would
+require taking over another worker's claim; when its own claim runs, it
+delivers again and the receiving side drops the replay. The same is true
+when two workers claim effects for one actor concurrently. Both runtimes
+share this behavior, and the Ruby suite pins it with a race test: order
+holds, duplicates apply nothing, and every effect completes.
+
 ## Retry budget and offline tolerance
 
 A raised delivery follows the effect retry policy: `max_attempts` (default