Issue #3484587: Clarify loadMultiple return behavior for invalid IDs

Updated the docblock of loadMultiple method to specify that invalid IDs will not be included in the returned array.

Closes #3484587

core/lib/Drupal/Core/Entity/EntityStorageInterface.php

@@ -41,8 +41,8 @@ public function resetCache(?array $ids = NULL);
    *   An array of entity IDs, or NULL to load all entities.
    *
    * @return \Drupal\Core\Entity\EntityInterface[]
-   *   An array of entity objects indexed by their IDs. Returns an empty array
-   *   if no matching entities are found.
+   *   An array of successfully loaded objects indexed by their IDs.
+   *   Returns an empty array if no matching entities are found.
    */
   public function loadMultiple(?array $ids = NULL);