kiji-express: EXP-317. Touch up TimeRange scaladocs.

issue: https://jira.kiji.org/browse/EXP-317
review: https://review.kiji.org/r/1404
commit: https://github.com/kijiproject/kiji-express/commit/4914180bb803c0da3b3f2b2b4a80303eab584fe1