Auto merge of #66238 - ehuss:stabilize-rustdoc-edition, r=GuillaumeGomez

Auto merge of #66238 - ehuss:stabilize-rustdoc-edition, r=GuillaumeGomez

rustdoc: Stabilize `edition` annotation.

The rustdoc `edition` annotation is currently ignored on stable. This means that the tests will be ignored, unless there is a `rust` annotation, then it will use the global edition. I suspect this was just an oversight during the edition stabilization, but I don't know. Example:

```rust
/// ```edition2018
/// // This code block was ignored on stable.
/// ```

/// ```rust,edition2018
/// // This code block would use whatever edition is passed on the command line.
/// ```
```

AFAIK, it is not possible to write a test that verifies stable behavior, as all tests appear to set RUSTC_BOOTSTRAP which forces all tests to run as "nightly", even on a stable release.

Closes #65980