Add links to helpful external resources.

Doc/library/functions.rst

@@ -1104,10 +1104,12 @@ available.  They are listed here in alphabetical order.
    *reverse* is a boolean value.  If set to ``True``, then the list elements are
    sorted as if each comparison were reversed.
 
-   In general, the *key* and *reverse* conversion processes are much faster than
-   specifying an equivalent *cmp* function.  This is because *cmp* is called
-   multiple times for each list element while *key* and *reverse* touch each
-   element only once.
+   In general, the *key* and *reverse* conversion processes are much faster
+   than specifying an equivalent *cmp* function.  This is because *cmp* is
+   called multiple times for each list element while *key* and *reverse* touch
+   each element only once.  To convert an old-style *cmp* function to a *key*
+   function, see the `CmpToKey recipe in the ASPN cookbook
+   <http://code.activestate.com/recipes/576653/>`_\.
 
    .. versionadded:: 2.4
 

Doc/library/math.rst

@@ -100,6 +100,10 @@ Number-theoretic and representation functions
    occasionally double-round an intermediate sum causing it to be off in its
    least significant bit.
 
+   For further discussion and two alternative approaches, see the `ASPN cookbook
+   recipes for accurate floating point summation
+   <http://code.activestate.com/recipes/393090/>`_\.
+
    .. versionadded:: 2.6