Document standard for mixed predicate test data

[jshook]

We need to be able to support some additional test data requirements, such as:

1. filtering by equality predicate
2. filtering by gt predicate
3. filtering by lt predicate

We also would like to be able to have datasets which allow us to synthesize fan-out queries such as "where value in ". We can do this with a data set which has the same query vectors applied to many vector sub-spaces. This is distinct from the case of simply having a single query vector over a single vector subspace.

In either case, it would be nice to document how this would work and provide support to nbvectors to spot check all of the documented and supported forms.