Rollup merge of #89596 - GuillaumeGomez:implicit-doc-cfg, r=jyn514

Make cfg imply doc(cfg)

This is a reopening of #79341, rebased and modified a bit (we made a lot of refactoring in rustdoc's types so they needed to be reflected in this PR as well):

 * `hidden_cfg` is now in the `Cache` instead of `DocContext` because `cfg` information isn't stored anymore on `clean::Attributes` type but instead computed on-demand, so we need this information in later parts of rustdoc.
 * I removed the `bool_to_options` feature (which makes the code a bit simpler to read for `SingleExt` trait implementation.
 * I updated the version for the feature.

There is only one thing I couldn't figure out: [this comment](https://github.com/rust-lang/rust/pull/79341#discussion_r561855624)

> I think I'll likely scrap the whole `SingleExt` extension trait as the diagnostics for 0 and >1 items should be different.

How/why should they differ?

EDIT: this part has been solved, the current code was fine, just needed a little simplification.

cc `@Nemo157`
r? `@jyn514`

Original PR description:

This is only active when the `doc_cfg` feature is active.

The implicit cfg can be overridden via `#[doc(cfg(...))]`, so e.g. to hide a `#[cfg]` you can use something like:

```rust
#[cfg(unix)]
#[doc(cfg(all()))]
pub struct Unix;
```

By adding `#![doc(cfg_hide(foobar))]` to the crate attributes the cfg `#[cfg(foobar)]` (and _only_ that _exact_ cfg) will not be implicitly treated as a `doc(cfg)` to render a message in the documentation.

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