Rollup merge of #142987 - lolbinarycat:rustdoc-non_exhaustive-enum-v-142599, r=GuillaumeGomez

rustdoc: show attributes on enum variants

mostly for #[non_exhaustive]

unsure if there's any attributes we should take care to *not* include, it could use `render_code_attribute` and `is_non_exhaustive` instead, if that is a concern.

fixes rust-lang/rust#142599

Author: matthiaskrgr