Issue #2170415 by Xano: Fix docs for plugin discovery interface

core/lib/Drupal/Component/Plugin/Discovery/DiscoveryInterface.php

@@ -19,7 +19,7 @@ interface DiscoveryInterface {
    * @param string $plugin_id
    *   A plugin id.
    *
-   * @return array|null
+   * @return mixed
    *   A plugin definition, or NULL if no definition was found for $plugin_id.
    */
   public function getDefinition($plugin_id);
@@ -27,7 +27,7 @@ public function getDefinition($plugin_id);
   /**
    * Gets the definition of all plugins for this type.
    *
-   * @return array
+   * @return mixed[]
    *   An array of plugin definitions (empty array if no definitions were
    *   found).
    */