Issue #3453551 by mstrelan: Fix return type of FormatterInterface::settingsSummary

(cherry picked from commit 8706c216)

core/lib/Drupal/Core/Field/FormatterInterface.php

@@ -34,7 +34,7 @@ public function settingsForm(array $form, FormStateInterface $form_state);
    * If an empty result is returned, a UI can still be provided to display
    * a settings form in case the formatter has configurable settings.
    *
-   * @return string[]
+   * @return array<string|\Stringable>
    *   A short summary of the formatter settings.
    */
   public function settingsSummary();