Document <svelte:document>

[acstll]

Should address #374. Or at least be a start.

I tried adding an example, but the REPL wasn't working as expected:

<svelte:document on:mouseenter='set({ entered: true })' />
	
<p>The mouse is {entered ? 'in' : 'out'}</p>

<script>
	export default {
		data() {
			return { entered: false }
		}
	}
</script>

Maybe that's because how the REPL is built?

[Rich-Harris]

thank you!