Add some more "handler" docs

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

docs/data/batch/native/custorder.rst

@@ -0,0 +1,13 @@
+
+Customer Order Batch
+====================
+
+**Batch Type Key:** ``custorder``
+
+**Purpose:** Create and/or process a customer order.
+
+**Default Handler:** :class:`~rattail:rattail.batch.custorder.CustomerOrderBatchHandler`
+
+As of this writing, the only actual use for this batch is to create a
+*new* customer order, specifically in the context of a so-called
+"case" or "special" order.