Tweak worksheet_update() of ordering batch view, to leverage handler

specifically this is to make use of handler's `update_row_quantity()` method,
when user enters new order quantities via worksheet

docs/api/views/purchasing.ordering.rst

@@ -0,0 +1,13 @@
+
+``tailbone.views.purchasing.ordering``
+======================================
+
+.. automodule:: tailbone.views.purchasing.ordering
+
+.. autoclass:: OrderingBatchView
+
+   .. autoattribute:: model_class
+
+   .. autoattribute:: default_handler_spec
+
+   .. automethod:: worksheet_update