Add some more "handler" docs

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

docs/data/batch/native/handheld.rst

@@ -0,0 +1,17 @@
+
+Handheld Batch
+==============
+
+**Batch Type Key:** ``handheld``
+
+**Purpose:** Process a basic / generic product batch from a handheld device.
+
+**Default Handler:** :class:`~rattail:rattail.batch.handheld.HandheldBatchHandler`
+
+This batch can read from a variety of file formats, which correspond
+to particular types of handheld devices.  The file is expected to have
+a UPC or barcode field, and can optionally have case and/or unit
+quantities for each.
+
+Executing this batch usually just "converts" it into either an
+:doc:`inventory` or a :doc:`labels`.