Use async save in doc

qutil/io.py

@@ -400,10 +400,13 @@ class AsyncDatasaver:
     >>> datasaver.shutdown()
 
     The datasaver can also be used as a context manager, in which case
-    cleanup is automatically taken care of.
+    cleanup is automatically taken care of. For syntactic sugar, calling
+    the datasaver has the same effect as
+    :meth:`AsyncDatasaver.save_async`.
 
     >>> with AsyncDatasaver() as datasaver:
-    ...     datasaver.save_sync(tmpdir / 'foo.npz', data=data)
+    ...     datasaver(tmpdir / 'foo.npz', data=data/2)
+    ...     datasaver(tmpdir / 'bar.npz', data=2*data)
 
     """
     def __init__(self, pickle_lib: Literal['pickle', 'dill'] = 'pickle', compress: bool = False):