[fullscreen] Simplify to "unfullscreen exitDoc" in "exit fullscreen"? (#14)

https://fullscreen.spec.whatwg.org/#exit-fullscreen

At the very end, it seems like "Let element be exitDoc's fullscreen element" and "Unfullscreen element within exitDoc" ought to be equivalent to "unfullscreen exitDoc" given the current definition of [collect ancestors](https://fullscreen.spec.whatwg.org/#collect-ancestors).

As written, a reader (like me) might otherwise wonder if *exitDoc* could have any fullscreen element after the steps or not.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fullscreen/issues/14