docs: improve description for Default Controller in Auto-Routing Legacy

Author: kenjis

user_guide_src/source/incoming/controllers.rst

@@ -456,12 +456,17 @@ Your method will be passed URI segments 3 and 4 (``'sandals'`` and ``'123'``):
 
 .. literalinclude:: controllers/014.php
 
-Defining a Default Controller
-=============================
+Default Controller (Legacy)
+===========================
 
-CodeIgniter can be told to load a default controller when a URI is not
-present, as will be the case when only your site root URL is requested. Let's try it
-with the ``Helloworld`` controller.
+The Default Controller is a special controller that is used when a URI end with
+a directory name or when a URI is not present, as will be the case when only your
+site root URL is requested.
+
+Defining a Default Controller (Legacy)
+--------------------------------------
+
+Let's try it with the ``Helloworld`` controller.
 
 To specify a default controller open your **app/Config/Routes.php**
 file and set this variable:
@@ -480,7 +485,7 @@ see the "Hello World" message.
 .. note:: The line ``$routes->get('/', 'Home::index');`` is an optimization that you will want to use in a "real-world" app. But for demonstration purposes we don't want to use that feature. ``$routes->get()`` is explained in :doc:`URI Routing <routing>`
 
 For more information, please refer to the :ref:`routes-configuration-options` section of the
-:doc:`URI Routing <routing>` documentation.
+:ref:`URI Routing <routing-auto-routing-legacy-configuration-options>` documentation.
 
 Organizing Your Controllers into Sub-directories (Legacy)
 =========================================================

user_guide_src/source/incoming/routing.rst

@@ -757,6 +757,8 @@ and executes ``index()`` method with passing ``'1'`` as the first argument.
 
 See :ref:`Auto Routing (Legacy) in Controllers <controller-auto-routing-legacy>` for more info.
 
+.. _routing-auto-routing-legacy-configuration-options:
+
 Configuration Options (Legacy)
 ==============================
 
@@ -765,12 +767,18 @@ These options are available at the top of **app/Config/Routes.php**.
 Default Controller (Legacy)
 ---------------------------
 
+For Site Root URI (Legacy)
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 When a user visits the root of your site (i.e., example.com) the controller to use is determined by the value set by
 the ``setDefaultController()`` method, unless a route exists for it explicitly. The default value for this is ``Home``
 which matches the controller at **app/Controllers/Home.php**:
 
 .. literalinclude:: routing/047.php
 
+For Directory URI (Legacy)
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 The default controller is also used when no matching route has been found, and the URI would point to a directory
 in the controllers directory. For example, if the user visits **example.com/admin**, if a controller was found at
 **app/Controllers/Admin/Home.php**, it would be used.