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/netrc.rst b/Doc/library/netrc.rst
index 8a2f1c6..323fd69 100644
--- a/Doc/library/netrc.rst
+++ b/Doc/library/netrc.rst
@@ -10,6 +10,10 @@
 
 .. versionadded:: 1.5.2
 
+**Source code:** :source:`Lib/netrc.py`
+
+--------------
+
 The :class:`netrc` class parses and encapsulates the netrc file format used by
 the Unix :program:`ftp` program and other FTP clients.