Today, BlockJoinQuery can join from child docIDs up to parent docIDs.
EG this works well for product (parent) + many SKUs (child) search.
But the reverse, which BJQ cannot do, is also useful in some cases.
EG say you index songs (child) within albums (parent), but you want to
search and present by song not album while involving some fields from
the album in the query. In this case you want to wrap a parent query
(against album), joining down to the child document space.