Add documentation for `List`, `list_spacing`

[joelburget]

Without documentation it's difficult for me to understand the List constructor of block and the list_spacing type. In particular, I would expect List to hold 'attr block list (rather than list list).

The closest to an explanation I found is this block: https://github.com/ocaml/omd/blob/d38041aa00ffad3b41f14d72c048ad1d9d359cbd/src/html.ml#L150-L175

It looks to me like each item in the outer list gets mapped to an li, and items in the inner list get mapped to either an inline or block element, but the logic is a bit confusing. I'd appreciate a note in the docs to clear this up (including how this is parsed) 🙂