Add reference to enumerate() to indices example.

Doc/tutorial/controlflow.rst

@@ -104,8 +104,8 @@ increment (even negative; sometimes this is called the 'step')::
    >>> range(-10, -100, -30)
    [-10, -40, -70]
 
-To iterate over the indices of a sequence, combine :func:`range` and :func:`len`
-as follows::
+To iterate over the indices of a sequence, you can combine :func:`range` and
+:func:`len` as follows::
 
    >>> a = ['Mary', 'had', 'a', 'little', 'lamb']
    >>> for i in range(len(a)):
@@ -117,6 +117,9 @@ as follows::
    3 little
    4 lamb
 
+In most such cases, however, it is convenient to use the :func:`enumerate`
+function, see :ref:`tut-loopidioms`.
+
 
 .. _tut-break: