Update API docs and demos for recent worker improvements.

>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.

This will happen only if you create more and more ZipReaders and don't release them. Why you want to do this?

>If .close() is called, then the behavior is deterministic, and one can reason about it.

"Deterministic" doesn't imply "easy". `malloc/free` are deterministic, but are not as easy to use as GC.

If someone feels .close() is good, just use it, and the doc already recommends to do so. But there is no reason to pretend that the GC won't work with zip.js and force users to pay excessive coding cost when the resource constraint is not a big problem in certain use cases. Just provide accurate informations and let users choose.