Add initial/basic docs for batches

that still needs a lot yet i'm sure, but is a start

docs/data/batch/handlers.rst

@@ -3,4 +3,17 @@
 Batch Handlers
 ==============
 
-TODO
+A "batch handler" contains logic to handle the entire life cycle of a batch.
+Meaning, the batch handler is what will actually create the batch, and update
+it per user edits, refresh it etc. and ultimately execute it.
+
+The UI exposes tools for interacting with the batch but ultimately the handler
+is what "acts on" the batch based on user input.
+
+See also :doc:`/base/handlers` for general info on handlers.
+
+Each type of batch will have precisely one "handler" associated with it.  For
+instance, there is a handler for inventory batches, and another for label
+batches, etc.  The app can override the handler for each batch type, should
+they need to customize behavior.  And of course if the app adds a new batch
+type then it must also provide a handler for it.