pythongh-120937: Reference weakref from the __del__ documentation (python#120940)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: chaen

Doc/reference/datamodel.rst

@@ -1667,6 +1667,8 @@ Basic customization
 
    It is not guaranteed that :meth:`__del__` methods are called for objects
    that still exist when the interpreter exits.
+   :class:`weakref.finalize` provides a straightforward way to register
+   a cleanup function to be called when an object is garbage collected.
 
    .. note::