Update API docs and demos for recent worker improvements.

When I reviewed your docs, I assumed that it was actually doing a bit more than `worker.terminate()`, but it seems that unlike the writer, the reader is quite simple. As such, document that calling `.close()` is optional, and only meant to free the resources as soon as possible.

- Add "immediately", so you get "and **immediately** terminate the associated workers"
- And replace the entire paragraph with "`When <clode>close</code> is not called, the worker may still be terminated when the reader is garbage-collected.`"