[Doc] State the Listen Stream Delivery Semantics

The Subscriptions page documented the acknowledgement-first contract but not what delivery means:
the stream carries only changes that happen after the acknowledgement, nothing is queued while
no stream is connected, and a dropped stream is gone together with anything it missed.
The Python SDK's subscriptions page states the same contract ("It is not a replay log").
Spelling it out gives client implementers the fetch-after-subscribing guidance and
makes the acknowledgement-window behavior explicit.

Author: koic

docs/_server/subscriptions.md

@@ -49,6 +49,13 @@ The first SSE event on the stream is the acknowledgement:
 }
 ```
 
+## Delivery Semantics
+
+A stream delivers only the changes that happen after its acknowledgement; it is not a replay log.
+Nothing is queued while no stream is connected, and a dropped stream is gone together with anything it missed.
+Clients fetch the current state after subscribing and treat each notification as a cue to refetch,
+rather than as a payload.
+
 ## Transport Support
 
 The stream is served on the Streamable HTTP modern path; stdio answers `-32601`.