Update API docs and demos for recent worker improvements.

No, `releaseSharedReaderWorker()` is just an *optimizition*, you are not forced to called it at every possible executing paths (so much easier to use than `close()`). If it is never called (very unlikely), there will be at most one worker left per-page, still tolerable. If the user forgets to clear the timer, nothing bad will happen, it just constantly sets `obj.zip._sharedReaderWorker` to null.