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/telnetlib.rst b/Doc/library/telnetlib.rst
index 925eccc..f6340a9 100644
--- a/Doc/library/telnetlib.rst
+++ b/Doc/library/telnetlib.rst
@@ -1,4 +1,3 @@
-
 :mod:`telnetlib` --- Telnet client
 ==================================
 
@@ -9,6 +8,10 @@
 
 .. index:: single: protocol; Telnet
 
+**Source code:** :source:`Lib/telnetlib.py`
+
+--------------
+
 The :mod:`telnetlib` module provides a :class:`Telnet` class that implements the
 Telnet protocol.  See :rfc:`854` for details about the protocol. In addition, it
 provides symbolic constants for the protocol characters (see below), and for the