docs: add changelog and upgrade

Author: kenjis

user_guide_src/source/changelogs/v4.5.0.rst

@@ -21,6 +21,16 @@ BREAKING
 Behavior Changes
 ================
 
+Nested Route Groups and Options
+-------------------------------
+
+Due to a bug fix, the behavior has changed so that options passed to the outer
+``group()`` are merged with the options of the inner ``group()``.
+See :ref:`Upgrading Guide <upgrade-450-nested-route-groups-and-options>` for details.
+
+Others
+------
+
 - **Logger:** The :php:func:`log_message()` function and the logger methods in
   ``CodeIgniter\Log\Logger`` now do not return ``bool`` values. The return types
   have been fixed to ``void`` to follow the PSR-3 interface.

user_guide_src/source/incoming/routing.rst

@@ -535,6 +535,8 @@ given route config options:
 
 .. literalinclude:: routing/027.php
 
+.. _routing-nesting-groups:
+
 Nesting Groups
 ==============
 

user_guide_src/source/installation/upgrade_450.rst

@@ -18,6 +18,37 @@ Mandatory File Changes
 Breaking Changes
 ****************
 
+.. _upgrade-450-nested-route-groups-and-options:
+
+Nested Route Groups and Options
+===============================
+
+A bug that prevented options passed to outer ``group()`` from being merged with
+options in inner ``group()`` has been fixed.
+
+Check and correct your route configuration as it could change the values of the
+options applied.
+
+For example,
+
+.. code-block:: php
+
+    $routes->group('admin', ['filter' => 'csrf'], static function ($routes) {
+        $routes->get('/', static function () {
+            // ...
+        });
+
+        $routes->group('users', ['namespace' => 'Users'], static function ($routes) {
+            $routes->get('/', static function () {
+                // ...
+            });
+        });
+    });
+
+Now the ``csrf`` filter is executed for both the route ``admin`` and ``admin/users``.
+In previous versions, it is executed only for the route ``admin``.
+See also :ref:`routing-nesting-groups`.
+
 Method Signature Changes
 ========================