Update 3. Demonstrate ability to customize request routing.md

Author: simpleadm

2. Request Flow Processing/3. Demonstrate ability to customize request routing.md

@@ -41,13 +41,17 @@ reference route by ID and add own module "before" original module.
 
 ### How do you handle custom 404 pages?
 
-1. If front controller catches NotFoundException, it changes action name *"noroute"* and continues loop.
+1. If [front controller](https://github.com/magento/magento2/blob/2.2-develop/lib/internal/Magento/Framework/App/FrontController.php#L61-L65) catches [\Magento\Framework\Exception\NotFoundException](https://github.com/magento/magento2/blob/2.2-develop/lib/internal/Magento/Framework/Exception/NotFoundException.php), it changes action name *"noroute"* and continues loop.
    E.g. catalog/product/view/id/1 throws NotFoundException. catalog/product/noroute is checked.
 
 1. If standard router recognizes front name but can't find controller, it tries to find *"noroute"*
    action from last checked module.
    E.g. catalog/brand/info controller doesn't exist, so catalog/brand/noroute will be checked.
+   
+   [\Magento\Framework\App\Router\Base::getNotFoundAction](https://github.com/magento/magento2/blob/2.2-develop/lib/internal/Magento/Framework/App/Router/Base.php#L237)
 
 1. If all routers didn't match, default controller provides two opportunities:
-  - set default 404 route in admin config `web/default/no_route`
-  - register custom handler in noRouteHandlerList
+  - set default 404 route in admin config `web/default/no_route` (see: [\Magento\Framework\App\Router\NoRouteHandler::process](https://github.com/magento/magento2/blob/2.2-develop/lib/internal/Magento/Framework/App/Router/NoRouteHandler.php#L34))
+  - register custom handler in [noRouteHandlerList](https://github.com/magento/magento2/blob/2.2-develop/lib/internal/Magento/Framework/App/Router/NoRouteHandlerList.php)
+    * backend (sortOrder: 10) [Magento\Backend\App\Router\NoRouteHandler](https://github.com/magento/magento2/blob/2.2-develop/app/code/Magento/Backend/App/Router/NoRouteHandler.php#L44) -> `adminhtml/noroute/index`
+    * default (sortOrder: 100) [Magento\Framework\App\Router\NoRouteHandler]((https://github.com/magento/magento2/blob/2.2-develop/lib/internal/Magento/Framework/App/Router/NoRouteHandler.php)