bpo-39429: Add a new "Python Development Mode" doc page (GH-18132)

diff --git a/Doc/library/asyncio-dev.rst b/Doc/library/asyncio-dev.rst
index 101e781..ff51c4f 100644
--- a/Doc/library/asyncio-dev.rst
+++ b/Doc/library/asyncio-dev.rst
@@ -25,7 +25,7 @@
 
 * Setting the :envvar:`PYTHONASYNCIODEBUG` environment variable to ``1``.
 
-* Using the :option:`-X` ``dev`` Python command line option.
+* Using the :ref:`Python Development Mode <devmode>`.
 
 * Passing ``debug=True`` to :func:`asyncio.run`.