Specified "onlost" attribute of WakeLock

I think we should simplify the language a little bit. Maybe: 

```HTML
 The <dfn id="widl-WakeLock-onlost"><code>onlost</code></dfn> <a>event 
 handler</a> provides a means for scripts of  a <a>top-level browsing context</a> to be notified 
if a <a>wake lock</a> is <a>lost</a>.
```