asyncio doc: remove reference to _DEBUG (now replaced with PYTHONASYNCIODEBUG

env var), document the default debug mode

Doc/library/asyncio-dev.rst

@@ -98,7 +98,6 @@ imported before the flag value can be changed.
 Example with the bug::
 
     import asyncio
-    asyncio.tasks._DEBUG = True
 
     @asyncio.coroutine
     def test():

Doc/library/asyncio-eventloop.rst

@@ -558,7 +558,7 @@ Debug mode
 
 .. method:: BaseEventLoop.get_debug()
 
-   Get the debug mode (:class:`bool`) of the event loop.
+   Get the debug mode (:class:`bool`) of the event loop, ``False`` by default.
 
 .. method:: BaseEventLoop.set_debug(enabled: bool)