[racket] Scribble and itemlists

I'm not saying "omg this is impossible!"

I'm pointing out that there's more work to do than updating some
Scribble structures. There are a few renderers that would need to be
updated, and in a way that gives satisfactory results in the messy
real world.

The HTML render should still work well in browsers and in tools.
(Good news: Eli says the `start` attribute is old and widely
supported.)

The markdown render should still work well in human readers' brains
and in tools.

> Of course, you can always just drop down to HTML in Markdown, so it’s not a big deal.

It depends how you're using the Scribble-generated markdown. If all
ordered lists are now rendered using HTML, that might be OK for
markdown-consuming tools, but not very good for markdown-reading
humans.

(Also it's not really the case you can always drop to HTML, anywhere.
There are official restrictions; also de facto implementation limits.
So to be safe for tools, an ordered list would probably need _all_ its
child elements rendered only as HTML. That entire block would read
like HTML not markdown.)

Of course, it may be that hardly anyone is using the
Scribble->markdown renderer, in which case it's moot. :)

It might also be fine to doc this away: "Hey if you try to render
non-contiguous ordered lists as markdown, it won't work. So don't try
that." :)