Add note about union types logic

Author: n.gnato

src/FreeElephants/JsonApi/DTO/AbstractDocument.php

@@ -19,6 +19,17 @@ final public function __construct(array $payload)
             /** @var \ReflectionUnionType $reflectionType */
             $dataClassName = $reflectionType->getTypes()[0]->getName();
         }
+
+        /**
+         * In cases like:
+         * `public array|Example\ResourceObjectExt|Example\ResourceObject $data;`
+         * ReflectionUnionType::getTypes() return types in next orders:
+         * - Example\ResourceObjectExt
+         * - Example\ResourceObject
+         * - array
+         *
+         * This exception is [can] not covered with test. But this behavior not documented at https://www.php.net/manual/en/reflectionuniontype.gettypes.php
+         */
         if ($dataClassName !== 'array') {
             $data = new $dataClassName($payload['data']);
         } else {