Auto merge of #10287 - Nilstrieb:docs-semver-check-run-fail, r=ehuss
Auto merge of #10287 - Nilstrieb:docs-semver-check-run-fail, r=ehuss

Add `run-fail` to semver-check for docs

I encountered this missing feature in #10276 and therefore added it here in this separate PR.

If the breaking change does not involve a compilation error but a change in runtime behaviour, you can add `run-fail` to the codeblock. The "before" code must return exit code 0, and the "after" code must be nonzero (like a panic).

Example case that I tested (ignore the trailing dot, it's for github markdown to not hate me)

```
```rust,ignore,run-fail
// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub fn foo() {}
///////////////////////////////////////////////////////////
// After
pub fn foo() {
    panic!("hey!");
}
///////////////////////////////////////////////////////////
// Example usage that will break.
fn main() {
    updated_crate::foo();
}
```.
```

rust-lang/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,rustyhorde/cargo,nipunn1313/cargo,rustyhorde/cargo,rust-lang/cargo,rust-lang/cargo,eminence/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo