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/sndhdr.rst b/Doc/library/sndhdr.rst
index 01a3917..f36df68 100644
--- a/Doc/library/sndhdr.rst
+++ b/Doc/library/sndhdr.rst
@@ -1,4 +1,3 @@
-
 :mod:`sndhdr` --- Determine type of sound file
 ==============================================
 
@@ -11,6 +10,10 @@
    single: A-LAW
    single: u-LAW
 
+**Source code:** :source:`Lib/sndhdr.py`
+
+--------------
+
 The :mod:`sndhdr` provides utility functions which attempt to determine the type
 of sound data which is in a file.  When these functions are able to determine
 what type of sound data is stored in a file, they return a tuple ``(type,