Add some more "handler" docs

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

docs/data/batch/native/purchase.rst

@@ -0,0 +1,27 @@
+
+Purchase Batch
+==============
+
+**Batch Type Key:** ``purchase``
+
+**Purpose:** Provides workflows for ordering, receiving, and invoice costing.
+
+**Default Handler:** :class:`~rattail:rattail.batch.purchase.PurchaseBatchHandler`
+
+There are 3 "modes" for this type of batch, meant to correspond to the
+life-cycle of a purchase order:
+
+- Ordering
+- Receiving
+- Costing
+
+Depending on the mode, data for the batch may come from any of:
+
+- PO file or invoice file from the vendor
+- purchase order data from the POS system
+- directly entered via the Rattail app
+
+Execution of this type of batch will also depend on the mode.  The
+default handler may just update Rattail data (e.g. ``purchase`` table)
+according to the batch.  If applicable, you may want a custom handler
+so it updates your POS instead.