rustdoc: fold fields for enum struct variants into a docblock
Per discussion in #37500, this PR updates the enum rendering code to wrap variants with named struct fields in a `docblock` span that is hidden automatically upon load of the page. This gives struct variant fields a clean separation from other enum variants, giving a boost to the readability of such documentation. Preview output is available [on the issue page](https://github.com/rust-lang/rust/issues/37500#issuecomment-
260069269), but for the sake of completeness I'll include the images here again.
![rustdoc struct enum variant 4 part 1](https://cloud.githubusercontent.com/assets/
5217170/
20231925/
96160b7e-a82a-11e6-945b-
bbba95c5e4bc.PNG)
When you initially load the page, there's a line under variants with struct fields letting you know you can click to expand the listing.
![rustdoc struct enum variant 4 part 2](https://cloud.githubusercontent.com/assets/
3050060/
20232067/
1dc63266-a866-11e6-9555-
8fb1c8afdcec.png)
If you click to expand, the header and table unfold into a nicely-indented listing.
If you want to take a look in your own browser and screen size, [I've got this version hosted on my server](https://shiva.icesoldier.me/doctest/doctest/enum.OldTopicRemoval.html).
Fixes #37500
r? @GuillaumeGomez