docs(gateway): reflow the mark_async docstring to 80 columns

PROOF: whitespace-only reflow of the previous commit's paragraph; longest
line 78 columns, ruff clean.

gateway/platforms/helpers.py

@@ -176,8 +176,9 @@ class ThreadParticipationTracker:
         ``_save`` ends in ``atomic_json_write`` -> ``os.replace``, whose
         duration is unbounded under filesystem pressure.  Every caller of this
         tracker sits on an inbound-message coroutine in the Discord and Matrix
-        adapters, so the rename must not be paid inline on the event loop -- it stalls every
-        other adapter's polling and every in-flight turn for as long as it runs.
+        adapters, so the rename must not be paid inline on the event loop -- it
+        stalls every other adapter's polling and every in-flight turn for as
+        long as it runs.
         """
         if self._remember(thread_id):
             await asyncio.to_thread(self._save)