Lightly modernize LegalpadDocumentQuery

Summary: Ref T13024. Updates LegalpadDocumentQuery to use slightly more modern constructions.

Test Plan: Queried for Legalpad documents, got the same results.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13024

Differential Revision: https://secure.phabricator.com/D18785