Rollup merge of #140216 - t5kd:master, r=tgross35

Document that "extern blocks must be unsafe" in Rust 2024

The [documentation on `extern`](https://doc.rust-lang.org/std/keyword.extern.html) contains the following code sample:
```rust
#[link(name = "my_c_library")]
extern "C" {
    fn my_c_function(x: i32) -> bool;
}
```

Due to #123743, attempting to compile such code with the 2024 edition of Rust fails:
```
error: extern blocks must be unsafe
```

This PR extends the `extern` documentation with a brief explanation of the new requirement. It also adds the missing `unsafe` keyword to the code sample, which should be compatible with rustc since v1.82.

**Related docs:**
- https://doc.rust-lang.org/reference/items/external-blocks.html
- https://doc.rust-lang.org/edition-guide/rust-2024/unsafe-extern.html

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