Clarify that Entity->delete() doesn't actually do anything

When deleting an entity that uses the Admin UI, EntityDefaultUIController->applyOperation() does a direct entity_delete() which bypasses any custom logic set in Entity->delete().

Although this apparently works as designed, as per #1265772: $entity->delete() doesn't behave like $wrapper->delete(), it should be made clearer in the documentation.

Also, it raises a question, why does Entity->delete() exist if when using the admin UI its completely ignored?