document IOBase.__del__'s behavior (closes #21764)

Patch from Nikolaus Rath.

Doc/library/io.rst

@@ -353,6 +353,12 @@ I/O Base Classes
       is usual for each of the lines provided to have a line separator at the
       end.
 
+   .. method:: __del__()
+
+      Prepare for object destruction. :class:`IOBase` provides a default
+      implementation of this method that calls the instance's
+      :meth:`~IOBase.close` method.
+
 
 .. class:: RawIOBase