Update API docs and demos for recent worker improvements.

A page in a subdomain can change `document.domain` to a upper one, though I don't known what's the impact on web workers. Except this, these may be some use cases for using absolute URLs with `workerScriptsPath`:

* Users already resolved the url for workerScriptsPath by themselves, it doesn't make sense to convert it to a path.
* Users are writing a privileged app (e.g. browser extension, packged web app), so cross-origin web worker is allowed.

My main argument is still that the meaning of "path" is ambiguous: it is not a filesystem path, but a path part of a URL - you still have to refer to "URL" to explain "path" clearly, so why not just call it "URL"?