Improve PHPdoc / IDE autocomplete for config tree builder

src/Symfony/Component/Config/Definition/Builder/TreeBuilder.php

@@ -51,6 +51,9 @@ class TreeBuilder implements NodeParentInterface
         return $this->root = $builder->node($name, $type)->setParent($this);
     }
 
+    /**
+     * @return NodeDefinition|ArrayNodeDefinition The root node (as an ArrayNodeDefinition when the type is 'array')
+     */
     public function getRootNode(): NodeDefinition
     {
         if (null === $this->root) {