rustdoc: properly nest markup within enum variant lists (fixes #6605)

This splits up multi-line annotations for individual enum variants
and prefixes each but the first with four spaces so that pandoc will
recognize everything as belonging to the same list item.

Since that introduces <p> tags for some list items, I've gone ahead
and inserted blank lines after each list item so that consistently
get <p> tags for all <li>s documenting variants. It's a bit less
compact now but still tolerable, I think.