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.dom.pulldom.rst b/Doc/library/xml.dom.pulldom.rst
index 80a91b8..bad0daa 100644
--- a/Doc/library/xml.dom.pulldom.rst
+++ b/Doc/library/xml.dom.pulldom.rst
@@ -1,4 +1,3 @@
-
 :mod:`xml.dom.pulldom` --- Support for building partial DOM trees
 =================================================================
 
@@ -9,6 +8,10 @@
 
 .. versionadded:: 2.0
 
+**Source code:** :source:`Lib/xml/dom/pulldom.py`
+
+--------------
+
 :mod:`xml.dom.pulldom` allows building only selected portions of a Document
 Object Model representation of a document from SAX events.