Combined Django DEFAULT_LOGGING with user LOGGING config

Refs #18993.

docs/topics/logging.txt

@@ -192,6 +192,8 @@ There are two other logging calls available:
 * ``logger.exception()``: Creates an ``ERROR`` level logging
   message wrapping the current exception stack frame.
 
+.. _configuring-logging:
+
 Configuring logging
 ===================
 
@@ -216,6 +218,14 @@ handlers, filters and formatters that you want in your logging setup,
 and the log levels and other properties that you want those components
 to have.
 
+Prior to Django 1.5, the :setting:`LOGGING` setting overwrote the :ref:`default
+Django logging configuration <default-logging-configuration>`. From Django
+1.5 forward, the project's logging configuration is merged with Django's
+defaults, hence you can decide if you want to add to, or replace the existing
+configuration. To completely override the default configuration, set the
+``disable_existing_loggers`` key to True in the :setting:`LOGGING`
+dictConfig. Alternatively you can redefine some or all of the loggers.
+
 Logging is configured as soon as settings have been loaded
 (either manually using :func:`~django.conf.settings.configure` or when at least
 one setting is accessed). Since the loading of settings is one of the first
@@ -535,3 +545,15 @@ logging module.
                 'class': 'django.utils.log.AdminEmailHandler'
             }
         },
+
+.. _default-logging-configuration:
+
+Django's default logging configuration
+======================================
+
+By default, Django configures the ``django.request`` logger so that all messages
+with ``ERROR`` or ``CRITICAL`` level are sent to :class:`AdminEmailHandler`, as
+long as the :setting:`DEBUG` setting is set to ``False``.
+
+All messages reaching the ``django`` catch-all logger are discarded
+(sent to ``NullHandler``).