kiji-express: EXP-406. Update scaladocs for the org.kiji.express.flow package.

issue: https://jira.kiji.org/browse/EXP-406
review: https://review.kiji.org/r/1576
commit: https://github.com/kijiproject/kiji-express/commit/475f1916160536ba0508758e082cb0c3499f106f