Document the underlying data model for DataViews

The DataView documentation (https://docs.enthought.com/pyface/data_view.html) dives straight into indexing details after a brief overview, but doesn't explain what the (abstract) data model being indexed is. It's difficult to make sense of the indexing discussion without first having a mental model of the type of data that's being indexed; "hierarchical and non-hierarchical tabular data" isn't really enough here. Some extra words or explanation, or possibly even an example or two, would be really helpful.