minor #30817 [Config] Improve PHPdoc / IDE autocomplete for config tree builder (Koc)

This PR was merged into the 4.2 branch.

Discussion
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[Config] Improve PHPdoc / IDE autocomplete for config tree builder

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

follow up of #27476 and #21047

Commits
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21f797714e 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) {