Add FullScreen to Documentation

[hallpell]

Currently the documentation for fullScreen is in the github description, but doesn't show up in the documentation on WoofJS. We should show that you need to do:
fullScreen = false
setBackdropSize(width, height)

I'm unsure if this should go into Background or Looks in the documentation. I think I prefer it in Background, but it currently is basically examples of Backgrounds, so it's somewhat likely to get mixed in. We should also ensure that the tags for it are set reasonably.