Added comment to indicate that WC_Order shouldn't be directly instantiated

includes/abstracts/abstract-wc-order.php

@@ -20,6 +20,9 @@ abstract class WC_Abstract_Order {
 
 	/**
 	 * Get the order if ID is passed, otherwise the order is new and empty.
+	 * This class should NOT be instantiated, but the get_order function or new WC_Order_Factory
+	 * should be used. It is possible, but the aforementioned are preferred and are the only
+	 * methods that will be maintained going forward.
 	 *
 	 * @access public
 	 * @param string $id (default: '')