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/xml.etree.elementtree.rst b/Doc/library/xml.etree.elementtree.rst
index def4669..a40fb65 100644
--- a/Doc/library/xml.etree.elementtree.rst
+++ b/Doc/library/xml.etree.elementtree.rst
@@ -1,4 +1,3 @@
-
 :mod:`xml.etree.ElementTree` --- The ElementTree XML API
 ========================================================
 
@@ -9,6 +8,10 @@
 
 .. versionadded:: 2.5
 
+**Source code:** :source:`Lib/xml/etree/ElementTree.py`
+
+--------------
+
 The :class:`Element` type is a flexible container object, designed to store
 hierarchical data structures in memory.  The type can be described as a cross
 between a list and a dictionary.