workflows: correspond object state names with docs

* Fix reference to `completed_objects` in the documentation (was:
  `final_objects` in code).

* Expose all other states via the `BibWorkflowEngine` interface and
  expand documentation.
You should add deprecation warning on `final_objects` if you would like to change the name. Why did you create `completed_objects` if the state is called `FINAL`?