N°4517 - PHP 8.1: Replace mixed return types with #[\ReturnTypeWillChange] PHP attributes to restore PHP 7.4 compatibility

Also add comment on previous #[\ReturnTypeWillChange]

core/designdocument.class.inc.php

@@ -77,6 +77,7 @@ class DesignDocument extends DOMDocument
 	 *
 	 * @return int
 	 */
+	// Return type union is not supported by PHP 7.4, we can remove the following PHP attribute and add the return type once iTop min PHP version is PHP 8.0+
 	#[\ReturnTypeWillChange]
 	public function save($filename, $options = 0)
 	{