diff --git a/core/lib/Drupal/Core/Field/TypedData/FieldItemDataDefinition.php b/core/lib/Drupal/Core/Field/TypedData/FieldItemDataDefinition.php
index 2cd755b..56dded4 100644
--- a/core/lib/Drupal/Core/Field/TypedData/FieldItemDataDefinition.php
+++ b/core/lib/Drupal/Core/Field/TypedData/FieldItemDataDefinition.php
@@ -86,7 +86,11 @@ public function getFieldDefinition() {
   /**
    * Sets the field item's field definition.
    *
-   * @param \Drupal\Core\Field\FieldDefinitionInterface
+   * Allows \Drupal\Core\Field\BaseFieldDefinition::__clone() to overwrite the
+   * itemDefinition property of the cloned field definition. Internal use only.
+   *
+   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
+   *   The new field definition to assign this item definition.
    *
    * @return static
    *   The object itself for chaining.