docs: Add asyncio source code links (GH-16640)

(cherry picked from commit f900064ac4b35226caad7502abc8a7e64f1c0e9d)

Co-authored-by: Kyle Stanley <aeros167@gmail.com>
diff --git a/Doc/library/asyncio-exceptions.rst b/Doc/library/asyncio-exceptions.rst
index b1e6ecf..7166d5c 100644
--- a/Doc/library/asyncio-exceptions.rst
+++ b/Doc/library/asyncio-exceptions.rst
@@ -7,6 +7,9 @@
 Exceptions
 ==========
 
+**Source code:** :source:`Lib/asyncio/exceptions.py`
+
+----------------------------------------------------
 
 .. exception:: TimeoutError