fix(route-alias): Add a note about aliasing 3rd-party routes

Author: welcoMattic

routing.rst

@@ -1403,7 +1403,7 @@ Instead of duplicating the original route, you can create an alias for it.
 
         class ProductController
         {
-            // "alias" named argument refers to the name of the alias you want to create.
+            // "alias" named argument indicates the name of the alias you want to create.
             // The alias will point to the actual route "product_show"
             #[Route('/product/{id}', name: 'product_show', alias: ['product_details'])]
             public function show(): Response
@@ -1452,6 +1452,15 @@ Instead of duplicating the original route, you can create an alias for it.
 In this example, both ``product_show`` and ``product_details`` routes can
 be used in the application and will produce the same result.
 
+.. note::
+
+    Using non-attributes formats (YAML, XML and PHP) is the only way
+    to define an alias pointing to a route that you don't own.
+
+    So that you can use your own route name for URL generation,
+    while actually using a route defined by a third-party bundle as the target of that URL generation,
+    as the 2 definitions are not required to be in the same config file (or even in the same format).
+
 .. _routing-alias-deprecation:
 
 Deprecating Route Aliases