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.
I believe `completed` is a more sensible name for a middle-level API. According to @jalavik that was also what was originally intended to be in the code. Good call on the deprecation warning - wil ldo.

I think a more human-friendly differentiation between `waiting` and `halted` could also be discussed.