kiji-express: EXP-323. Quick update to scaladocs on PagingSpec

issue: https://jira.kiji.org/browse/EXP-323
review: https://review.kiji.org/r/1413
commit: https://github.com/kijiproject/kiji-express/commit/ad170cf52f387d63f2a841db1ce6e6fc4e60e36a