kiji-express: EXP-321. Touch up scaladocs of ColumnInputSpec and its subclasses.

issue: https://jira.kiji.org/browse/EXP-321
review: https://review.kiji.org/r/1410
commit: https://github.com/kijiproject/kiji-express/commit/5ed6b56763d277a7cef07eaaa9fa63219bf7d242