Backport source links from 3.x.

Existing links have been updated to use the new reST role.  In some
files, I have also made cosmetic changes to the header.
diff --git a/Doc/library/cookie.rst b/Doc/library/cookie.rst
index de84568..480dffa 100644
--- a/Doc/library/cookie.rst
+++ b/Doc/library/cookie.rst
@@ -11,6 +11,9 @@
    3.0.  The :term:`2to3` tool will automatically adapt imports when converting
    your sources to 3.0.
 
+**Source code:** :source:`Lib/Cookie.py`
+
+--------------
 
 The :mod:`Cookie` module defines classes for abstracting the concept of
 cookies, an HTTP state management mechanism. It supports both simple string-only