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

diff --git a/Doc/library/asyncio-stream.rst b/Doc/library/asyncio-stream.rst
index 471e6e9..bb89971 100644
--- a/Doc/library/asyncio-stream.rst
+++ b/Doc/library/asyncio-stream.rst
@@ -6,6 +6,10 @@
 Streams
 =======
 
+**Source code:** :source:`Lib/asyncio/streams.py`
+
+-------------------------------------------------
+
 Streams are high-level async/await-ready primitives to work with
 network connections.  Streams allow sending and receiving data without
 using callbacks or low-level protocols and transports.