Update logging documentation (aio-libs#3243)

docs/logging.rst

@@ -28,8 +28,11 @@ configuring whole loggers in your application.
 Access logs
 -----------
 
-Access log by default is switched on and uses ``'aiohttp.access'``
-logger name.
+Access logs are enabled by default. If the
+:attr:``aiohttp.web.Application.debug`` is set, and the default logger
+``'aiohttp.access'`` is used, access logs will be output to :obj:`~sys.stderr`
+if no handlers are attached. Furthermore, if the default logger has no log
+level set, the log level will be set to :obj:`logging.DEBUG`.
 
 The log may be controlled by :meth:`aiohttp.web.AppRunner` and
 :func:`aiohttp.web.run_app`.