Update API docs and demos for recent worker improvements.

I thought so before, but this is not true, at least firefox and chrome (IE not tested yet) can GC inactive workers:
https://bugzilla.mozilla.org/show_bug.cgi?id=1103942

By "inactive" I mean there is no executing script, nor pending async IO operations and timers in a worker.

You can test yourself:
```
<!DOCTYPE html>
<html xmlns='http://www.w3.org/1999/xhtml'>
<head>
<meta charset="UTF-8" />
<title>worker and gc 2</title>
</head>
<body>
<button onclick="release();">release worker</button>
<script>
  var workerScript = 'data:text/javascript, addEventListener("message", function() {}); var a = new ArrayBuffer(300 * 1024 * 1024);';
  var worker = new Worker(workerScript);
  function release() {
    worker = null;
  }
</script>
</body>
</html>
```