Add some more "handler" docs

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

docs/data/batch/native/vendor_invoice.rst

@@ -0,0 +1,20 @@
+
+Vendor Invoice Batch
+====================
+
+**Batch Type Key:** ``vendor_invoice``
+
+**Purpose:** (DEPRECATED) Process a vendor invoice
+
+**Default Handler:** :class:`~rattail:rattail.batch.vendorinvoice.VendorInvoiceHandler`
+
+.. warning::
+   This batch type should be considered deprecated.  It still exists
+   and may be used, but that may not always be the case.  If possible
+   please use :doc:`purchase` instead.
+
+Data for this type of batch usually comes from a spreadsheet or
+similar file, as obtained directly from the vendor.
+
+Executing this batch does nothing at all by default; you would need a
+custom handler to make it useful.