Clarify file-closing example in tutorial (GH-11652)

Doc/tutorial/inputoutput.rst

@@ -322,6 +322,8 @@ equivalent :keyword:`try`\ -\ :keyword:`finally` blocks::
 
     >>> with open('workfile') as f:
     ...     read_data = f.read()
+
+    >>> # We can check that the file has been automatically closed.
     >>> f.closed
     True