…jyn514

Rename "stability" CSS class to "item-info" and combine `document_stability` with `document_short`

Follow-up of rust-lang#79300

The point of this PR is to make the CSS class more accurate since it's not only about stability anymore.

r? `@jyn514`

…ark-Simulacrum

BTreeMap: try to enhance various comments

All in internal documentation, propagating the "key-value pair" notation from public documentation.

r? `@Mark-Simulacrum`

…rror-output, r=jyn514

Improve rustdoc JS tests error output

It's pretty common when starting to add new tests for rustdoc-js to have issues to understand the errors. With this, it should make things a bit simpler. So now, in case of an error, it displays:

```
---- [js-doc-test] rustdoc-js/basic.rs stdout ----

error: rustdoc-js test failed!
failed to decode compiler output as json: line: {
output: Checking "basic" ... FAILED
==> Result not found in 'others': '{"path":"basic","name":"Fo"}'
Diff of first error:
{
    "path": "basic",
    - "name": "Fo",
    + "name": "Foo",
}
thread '[js-doc-test] rustdoc-js/basic.rs' panicked at 'explicit panic', src/tools/compiletest/src/json.rs:126:21
```

I think it was `@camelid` who asked about it a few days ago?

r? `@jyn514`

…=jyn514

Extend doc keyword feature by allowing any ident

Part of rust-lang#51315.

As suggested by `@danielhenrymantilla` in [this comment](rust-lang#51315 (comment)), this PR extends `#[doc(keyword = "...")]` to allow any ident to be used as keyword. The final goal is to allow (proc-)macro crates' owners to write documentation of the keywords they might introduce.

r? `@jyn514`

Cleanup: shorter and faster code