Rollup merge of #143665 - obi1kenobi:pg/doc-hidden-tests, r=aDotInTheVoid

[rustdoc-json] Add tests for `#[doc(hidden)]` handling of items.

Add tests which check:
- `#[doc(hidden)]` items are not present in rustdoc JSON output by default.
- Invoking rustdoc with `--document-hidden-items` makes `#[doc(hidden)]` items appear, and they show their `#[doc(hidden)]` status appropriately.

r? `@aDotInTheVoid`

Author: matthiaskrgr