docs(plex): correct the cull-yield fallback comment to name the real route 404s

Deep-review round 3: the only surviving finding — a comment-drift nit. The
fallback's cull-yield comment named the route's 'Not available yet' path, but a
culled-key segment request actually returns 'No active playback session' (the
substantive invariant — never a None that 404s as file-not-found — is unchanged).
Reworded to match the route. Concurrency + correctness came back clean.

backend/app/plex/stream.py

@@ -667,9 +667,11 @@ def start_session(
         )
         respawned = _spawn(False, False, replace_only=s)
         if respawned is None:
-            # Superseded/culled mid-fallback — yield the session registered now (a newer seek-restart), or
-            # our own now-dead session if the key was cap-culled (its segment requests degrade to the
-            # route's retryable "not available yet", never a dishonest "file not found" from a None).
+            # Superseded/culled mid-fallback — yield whoever is registered now (a newer seek-restart), or
+            # our own now-dead session if the key was cap-culled. Never None here: the POST /session route
+            # turns a None return into a "Media file not found on disk" 404 — dishonest for a file that
+            # exists. A (dead) session instead degrades to a retryable 404 ("No active playback session" /
+            # "Not available yet") the client re-requests.
             return current_session(key) or s
         s = respawned
         ready = wait_for(_seg(0), timeout=25.0, proc=s.proc)