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

diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index 25a3692..0029d94 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -1200,7 +1200,7 @@
 
    .. versionchanged:: 3.7
 
-      The new ``-X dev`` command line option can now also be used
+      The new :ref:`Python Development Mode <devmode>` can now also be used
       to enable the debug mode.
 
 .. seealso::