Add example for createNamed Form

The docs are not very clear where the FormFactory comes from and how to use this method

Author: wimme002

forms.rst

@@ -692,7 +692,19 @@ the choice is ultimately up to you.
 
     The form name is automatically generated from the type class name. If you want
     to modify it, use the :method:`Symfony\\Component\\Form\\FormFactoryInterface::createNamed` method.
-    You can even suppress the name completely by setting it to an empty string.
+    You can even suppress the name completely by setting it to an empty string::
+
+        // src/Controller/TaskController.php
+        use App\Form\TaskType;
+
+        public function new()
+        {
+            $task = ...;
+            $form = $this->container->get('form.factory')->createNamed('customName', TaskType::class, $task);
+
+            // ...
+        }
+
 
 Final Thoughts
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