Rollup merge of #125503 - aDotInTheVoid:rdj-keyword-attr, r=GuillaumeGomez

rustdoc-json: Add test for keywords with `--document-private-items`

Turns out this does work as-expected. I was worried that the rustdoc's clean would produce a `ItemKind::KeywordItem` for the module, and loose the module itself. But turns out we get this right.

Prompted by [this discussion on zulip](https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/Where.20to.20find.20keyword.20entries.20in.20JSON.20rustdoc)

r? `@GuillaumeGomez`

Author: matthiaskrgr