Revise "About" and "WorkflowManager" API Docs

Especially in adding better documentation of how to use the
WorkflowManager with len() and indexing and so on.

This also means that giving IndexedPiece objects to WorkflowManager is an
undocumented feature; I only left it because it's important for testing.
Further Improvements to WorkflowManager Docs; Remove _for_sc()

- Since we can't implement _for_sc() as a "modification," I removed it.
- Improved docs or other things.