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/symbol.rst b/Doc/library/symbol.rst
index 1735276..75a4792 100644
--- a/Doc/library/symbol.rst
+++ b/Doc/library/symbol.rst
@@ -1,4 +1,3 @@
-
 :mod:`symbol` --- Constants used with Python parse trees
 ========================================================
 
@@ -6,6 +5,9 @@
    :synopsis: Constants representing internal nodes of the parse tree.
 .. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
 
+**Source code:** :source:`Lib/symbol.py`
+
+--------------
 
 This module provides constants which represent the numeric values of internal
 nodes of the parse tree.  Unlike most Python constants, these use lower-case