Add some more "handler" docs

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

docs/data/batch/native/inventory.rst

@@ -0,0 +1,18 @@
+
+Inventory Batch
+===============
+
+**Batch Type Key:** ``inventory``
+
+**Purpose:** Process counts and adjustments for product inventory.
+
+**Default Handler:** :class:`~rattail:rattail.batch.inventory.InventoryBatchHandler`
+
+Data for this batch can come directly from file, or from a Handheld
+Batch etc.  It can contain counts for various products.
+
+Executing this batch with the default handler in place, will cause
+Rattail's own inventory counts to be updated according to the batch
+data.  Depending on your POS system it may be possible to push the
+updates to it instead, and if so you'd probably want to, since your
+POS is likely the authority for current inventory levels.