bpo-33649: Fix asyncio-dev (GH-9324)

diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index 19a0acd..718c322 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -1075,6 +1075,11 @@
 
    Set the debug mode of the event loop.
 
+   .. versionchanged:: 3.7
+
+      The new ``-X dev`` command line option can now also be used
+      to enable the debug mode.
+
 .. seealso::
 
    The :ref:`debug mode of asyncio <asyncio-debug-mode>`.