Issue #1126000 by yched, swentel, catch: Fixed Document that we don't support...

Issue #1126000 by yched, swentel, catch: Fixed Document that we don't support partial entity updates.

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

@@ -138,6 +138,9 @@ public function uriRelationships();
   /**
    * Saves an entity permanently.
    *
+   * When saving existing entities, the entity is assumed to be complete,
+   * partial updates of entities are not supported.
+   *
    * @return
    *   Either SAVED_NEW or SAVED_UPDATED, depending on the operation performed.
    *