fixed : add doc blocks to silence Symfony deprecations (#1922)

* fixed : add doc blocks to silence Symfony deprecations

Symfony 5.4 returning deprecations about 
```
User Deprecated: Method "Twig\Extension\ExtensionInterface::getFunctions()" might add "array" as a native return type declaration in the future.
Do the same in implementation "Nelmio\ApiDocBundle\Render\Html\GetNelmioAsset" now to avoid errors or add an explicit @return annotation to suppress this message.
```

* fixed : native hint

Render/Html/GetNelmioAsset.php

@@ -31,7 +31,7 @@ public function __construct(AssetExtension $assetExtension)
         $this->resourcesDir = __DIR__.'/../../Resources/public';
     }
 
-    public function getFunctions()
+    public function getFunctions(): array
     {
         return [
             new TwigFunction('nelmioAsset', $this, ['is_safe' => ['html']]),