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/warnings.rst b/Doc/library/warnings.rst
index b3c1994..dbcf148 100644
--- a/Doc/library/warnings.rst
+++ b/Doc/library/warnings.rst
@@ -9,6 +9,10 @@
.. versionadded:: 2.1
+**Source code:** :source:`Lib/warnings.py`
+
+--------------
+
Warning messages are typically issued in situations where it is useful to alert
the user of some condition in a program, where that condition (normally) doesn't
warrant raising an exception and terminating the program. For example, one