docs: improve explanations and add notes

Author: kenjis

user_guide_src/source/installation/upgrade_routing.rst

@@ -17,18 +17,22 @@ What has been changed
 - In CI4 the Auto Routing is disabled by default.
 - In CI4 the new more secure :ref:`auto-routing-improved` is introduced.
 - In CI4 the routing is no longer configured by setting the routes as array.
+- The Wildcard ``(:any)`` In CI3 will be the Placeholder ``(:segment)`` in CI4. The ``(:any)`` in CI4 matches multiple segements. See :ref:`URI Routing <routing-placeholder-any>`.
 
 Upgrade Guide
 =============
 
 1. If you use the Auto Routing in the same way as CI3, you need to enable :ref:`auto-routing-legacy`.
-2. The placeholder ``(:any)`` in CI3 will be ``(:segment)`` in CI4.
-3. You have to change the syntax of each routing line and append it in **app/Config/Routes.php**. For example:
+2. You have to change the syntax of each routing line and append it in **app/Config/Routes.php**. For example:
 
     - ``$route['journals'] = 'blogs';`` to ``$routes->add('journals', 'Blogs::index');``. This would map to the ``index()`` method in the ``Blogs`` controller.
-    - ``$route['product/(:any)'] = 'catalog/product_lookup';`` to ``$routes->add('product/(:segment)', 'Catalog::productLookup');``
+    - ``$route['product/(:any)'] = 'catalog/product_lookup';`` to ``$routes->add('product/(:segment)', 'Catalog::productLookup');``. Don't forget to replace ``(:any)`` with ``(:segment)``.
     - ``$route['login/(.+)'] = 'auth/login/$1';`` to ``$routes->add('login/(.+)', 'Auth::login/$1');``
 
+    .. note:: For backward compatibility, ``$routes->add()`` is used here. But we
+        strongly recommend to use :ref:`routing-http-verb-routes` like
+        ``$routes->get()`` instead of ``$routes->add()`` for security.
+
 Code Example
 ============
 
@@ -43,3 +47,7 @@ CodeIgniter Version 4.x
 Path: **app/Config/Routes.php**:
 
 .. literalinclude:: upgrade_routing/001.php
+
+.. note:: For backward compatibility, ``$routes->add()`` is used here. But we
+    strongly recommend to use :ref:`routing-http-verb-routes` like
+    ``$routes->get()`` instead of ``$routes->add()`` for security.