[Doc] Correct the SessionClosed admission-slot guarantee (#7636 Issue 22) (#7869)

Signed-off-by: yanbao1217 <yanliuwebsite493@gmail.com>
Co-authored-by: yanbao1217 <yanliuwebsite493@gmail.com>

vllm_omni/engine/duplex/session/manager.py

@@ -604,9 +604,12 @@ class DuplexSessionManager:
         The session keeps its admission slot (``_closing``) until stage cleanup
         succeeded; a failed cleanup is retried by the reaper. For an explicit
         close the ``session.closed`` event is emitted only after the cleanup
-        attempt, so a client that sees it can open a replacement session at
-        once (the runner used to emit it before the stage requests were
-        aborted, which let a prompt reopen hit ``resource_exhausted``).
+        attempt, in a ``finally``, including when the cleanup failed (the
+        runner used to emit it before the stage requests were aborted). A
+        client that sees the event can normally open a replacement session
+        right away, but after a failed cleanup the slot is still held until
+        the reaper succeeds, so a prompt reopen can be refused with
+        ``resource_exhausted``.
         """
         session = runner.session
         submitted = tuple(session.resource_request_ids(submitted=True))