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

This indents all but the first line of multi-line annotations for
individual enum variants 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.
auto merge of #6700 : ben0x539/rust/nestvariantdocs, r=thestinger

This indents all but the first line of multi-line annotations for individual enum variants 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.