This is the simplest case for OAK-3973. This is about the most basic case where a suggestion/spellcheck query with:
- single isdescendantnode() clause
- unambiguous index def (unlike OAK-3992)
- no union (unlike OAK-3993)
should work. That means that we should be able to work with:
- index on root and descendant clause on a sub-tree
- index on say "/a" and descendant clause also on "/a"
- index on say "/a" and descendant clause on "/a/b"