Add links from builtins module docs to built-in functions and constants docs

Doc/library/__builtin__.rst

@@ -8,7 +8,9 @@
 
 This module provides direct access to all 'built-in' identifiers of Python; for
 example, ``__builtin__.open`` is the full name for the built-in function
-:func:`open`.
+:func:`open`.  See :ref:`built-in-funcs` and :ref:`built-in-consts` for
+documentation.
+
 
 This module is not normally accessed explicitly by most applications, but can be
 useful in modules that provide objects with the same name as a built-in value,

Doc/library/constants.rst

@@ -1,3 +1,5 @@
+.. _built-in-consts:
+
 Built-in Constants
 ==================