step(n) docstring: raw-exchange ledger holds the final interval only

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CelerisLab/lbm/refinement_stepper.py

@@ -826,8 +826,8 @@ class FixedPatchStepper:
 
         All n intervals are enqueued in order on the one stream, and the host waits
         once, before returning; step(0) launches and waits for nothing. On return
-        the device work is complete and step_count, the raw-exchange ledger and an
-        open window cover all n intervals.
+        the device work is complete, step_count and an open window cover all n
+        intervals, and the raw-exchange ledger holds the final interval only.
 
         Asynchronous device errors surface at that single wait, so they are not
         attributed to the interval that caused them; call step(1) repeatedly to