Update API docs and demos for recent worker improvements.

> If it is never called (very unlikely),

Not unlikely at all, especially since you assumed that users will forget to call `.close()`

> there will be at most one worker left per-page, still tolerable.

One too many.

> If the user forgets to clear the timer, nothing bad will happen,

Resource leaks.

> it just constantly sets obj.zip._sharedReaderWorker to null.

And defeats the purpose of pooling, namely controlling/constraining the number of active workers in use by zip.js