diff --git a/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryInterface.php b/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryInterface.php
index 5b44bf9..64cab92 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryInterface.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryInterface.php
@@ -37,7 +37,7 @@ public function getDefinition($plugin_id, $exception_on_invalid = TRUE);
    *
    * @return mixed[]
    *   An array of plugin definitions (empty array if no definitions were
-   *   found).
+   *   found). Keys must be plugin IDs.
    */
   public function getDefinitions();