gh-133881: add forward reference to list.sort() in lambda expression tutorial (#133910)

Doc/tutorial/controlflow.rst

@@ -999,7 +999,8 @@ scope::
    43
 
 The above example uses a lambda expression to return a function.  Another use
-is to pass a small function as an argument::
+is to pass a small function as an argument.  For instance, :meth:`list.sort`
+takes a sorting key function *key* which can be a lambda function::
 
    >>> pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
    >>> pairs.sort(key=lambda pair: pair[1])