Document close() as the preferred mmap-release path

close() is the explicit API; __del__ is a GC safety net. Empty __del__
docstring previously left the intent implicit. Per review feedback.

Author: mmshad

kempnerforge/data/dataset.py

@@ -198,10 +198,11 @@ def _close_mmaps(self) -> None:
         self._mmaps.clear()
 
     def close(self) -> None:
-        """Explicit cleanup for callers that want to release mmaps before GC."""
+        """Release the underlying mmaps. Preferred path; do not rely on ``__del__``."""
         self._close_mmaps()
 
     def __del__(self) -> None:
+        """GC safety net only. Prefer explicit :meth:`close`."""
         with contextlib.suppress(Exception):
             self._close_mmaps()