Support "phriction.document.search" queries by "parentPaths" or "ancestorPaths"

Summary: Ref T13090. Ref T13077. This adds parentPaths and ancestorPaths constraints to phriction.document.query. These should be a little more usable than the internal slugPrefix / depth stuff -- that's technically more powerful, but requires callers to know more slug normalization rules. We could perhaps expose minDepth / maxDepth in the future.

Test Plan: Ran valid and invalid parentPaths and ancestorPaths queries for /, aaa/, AAA/, etc. Got sensible-seeming results.

Maniphest Tasks: T13090, T13077

Differential Revision: https://secure.phabricator.com/D19125