Better documentation on `num_workers`, `worker_work_ratio`

[ben-pr-p]

Hi!

Thanks for the library!

Could you write up some documentation on how those options work for ParallelStream.map?

My intuition says that as long as num_workers: 1, it's effectively not parallel, and worker_work_ratio is for managing the batch size passed to each worker for fine tuning acceptable parallel overhead?

[beatrichartz]

Yes you're right with your assumptions as well as the assertion that these could be documented better.

[Jdcorley]

Could I take this as a first-time open source contribution?

[beatrichartz]

@Jdcorley absolutely, have at it!