Add some more "handler" docs

app handler, batch handlers, native batch types, people handler

docs/data/batch/native/product.rst

@@ -0,0 +1,15 @@
+
+Product Batch
+=============
+
+**Batch Type Key:** ``product``
+
+**Purpose:** Generic batch to identify and act on a set of items.
+
+**Default Handler:** :class:`~rattail:rattail.batch.product.ProductBatchHandler`
+
+Data for this batch type usually comes from a simple file, or perhaps
+a grid query.  Executing it will usually create another type of batch
+using the current one only to define which items are involved.  For
+example by default it will create either a :doc:`labels` or a
+:doc:`pricing`.