Add implementation of FormTypeExtensionInterface::getExtendedTypes (#…

…1453)

This method is wanted by symfony >= 4.2 and will be required for
symfony >= 5.0

Fixes #1448

Form/Extension/DocumentationExtension.php

@@ -34,6 +34,11 @@ public function configureOptions(OptionsResolver $resolver)
 
     public function getExtendedType()
     {
-        return FormType::class;
+        return self::getExtendedTypes()[0];
+    }
+
+    public static function getExtendedTypes()
+    {
+        return [FormType::class];
     }
 }