workflows: object state names match docs

• Fixes reference to completed_objects in the documentation (was: final_objects in code).

• Exposes all other states via the BibWorkflowEngine interface and expand documentation.

Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Signed-off-by: Dimitrios Semitsoglou-Tsiapos <dsemitso@cern.ch>