kiji-express: EXP-389. Cleanup Scaladoc and constructor visibility around Builders

issue: https://jira.kiji.org/browse/EXP-389
review: https://review.kiji.org/r/1532
commit: https://github.com/kijiproject/kiji-express/commit/7493ee0975b4e4dc3d6ca129034ebb708021477b