Add some more "handler" docs

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

docs/base/handlers/batch.rst

@@ -0,0 +1,15 @@
+
+Batch Handlers
+==============
+
+There is a handler for each unique "type" of batch which exists in the
+app.  For instance the Pricing Batch has its own handler, and so does
+the Inventory Batch, and the Purchase Batch, and so on.
+
+See :doc:`/data/batch/handlers` for some more background on these.
+
+See :doc:`/data/batch/native/index` for a list of natively-supported
+batch types.  (Again, there is a handler for each type.)
+
+See also :class:`rattail:rattail.batch.handlers.BatchHandler` which is
+the base class for all batch handlers.