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.
In `utils` there are usually functions that could live in separate packages. `@staticproperty` and `@classproperty` could be in theory part of `functools` or similar package.