Update API docs and demos for recent worker improvements.

> > And defeats the purpose of pooling, namely controlling/constraining the number of active workers in use by zip.js
> 
> It does constrain the number of active workers to <= 1.

Not. when the timer is fired, the reference to the shared worker is discarded. In a long-running single page application, the workers will add up, up to the point where either the max number of workers is reached.

`.close()` already exists, it is more explicit, and it is deterministic unlike any timer-based approach that occasionally sets a variable to null.

If `.close()` is called, then the behavior is deterministic, and one can reason about it.