Only consider fullscreen elements' children for link hints

This PR stops us from showing link hints for elements concealed by fullscreening when an element is made fullscreen.

Fullscreen youtube videos are the best example of this; it's useful to be able to click the play/pause and fullscreen buttons, but we also show a lot of link hints for links on the underlying page, which we can't see.
The `document.documentElement` told XPath which document to search in, but yes, the specific element we passed was ignored and could have been anything within the document.

> Could this be harmless but unnecessary?

I don't understand what 'this' means in this context.