More type hints on some handler classes

src/Monolog/Handler/FormattableHandlerTrait.php

@@ -52,6 +52,8 @@ trait FormattableHandlerTrait
     /**
      * Gets the default formatter.
      *
+     * Overwrite this if the LineFormatter is not a good default for your handler.
+     *
      * @return FormatterInterface
      */
     protected function getDefaultFormatter(): FormatterInterface