Merge #7570 #7571

7570: Add doc gen to the `generate_enum_match_method` assist r=yoshuawuyts a=yoshuawuyts

Implements a small extension to https://github.com/rust-analyzer/rust-analyzer/pull/7562, generating default comments. I wasn't sure if this would fit the goals of Rust-Analyzer, so I chose to split it into a separate PR. This is especially useful when writing code in a codebase which uses `#![warn(missing_docs)]` lint, as many production-grade libraries do.

The comments we're generating here are similar to the ones found on [`Option::is_some`](https://doc.rust-lang.org/std/option/enum.Option.html#method.is_some) and [`Result::is_err`](https://doc.rust-lang.org/std/result/enum.Result.html#method.is_err). I briefly considered only generating these for `pub` types, but they seem small and unobtrusive enough that they're probably useful in the general case. Thanks!

## Example

__input__
```rust
pub(crate) enum Variant {
    Undefined,
    Minor, // cursor here
    Major,
}
```

__output__
```rust
pub(crate) enum Variant {
    Undefined,
    Minor,
    Major,
}

impl Variant {
    /// Returns `true` if the variant is [`Minor`].
    pub(crate) fn is_minor(&self) -> bool {
        matches!(self, Self::Minor)
    }
}
```

## Future Directions

This opens up the path to adding an assist for generating these comments on existing `is_` methods. This would make it both easy to document new code, and update existing code with documentation.

7571: Cleanup decl_check r=Veykril a=Veykril

bors r+

Co-authored-by: Yoshua Wuyts <[email protected]>
Co-authored-by: Lukas Wirth <[email protected]>