motion: mark MovingCircle.advance as a test helper, not the production path

Docstring only.

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

src/CelerisLab/body/motion.py

@@ -99,7 +99,12 @@ class MovingCircle:
         return self.centre(self.applied), self.centre(offset), velocity
 
     def advance(self, upload: Callable, step: Callable, synchronize: Callable) -> None:
-        """Upload the record, run one step and commit only after a successful sync.
+        """Single-circle transaction helper (unit tests): upload, step, sync, commit.
+
+        Not the production path: ``Simulation._run_moving`` batches all circles,
+        adds the ``no_donor`` gate and uses ``commit``/``fail`` directly.
+
+        Upload the record, run one step and commit only after a successful sync.
 
         Any exception after the upload is attempted leaves device geometry
         unknown: the circle is marked failed, never reports applied state again,