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.
Updating with changes and docstrings.

But in this case, should `func:unicodifier` `func:utf8ifier` be moved to `utils.py` as well?