Add doc gen to the generate_enum_match_method assist

[yoshuawuyts]

Implements a small extension to #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 and Result::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

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

output

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.

[matklad]

bors d+

Yeah, it's unclear if we want this long term, and in which shape exactly. But this indeed seems unobtrusive, and we wouldn't know if we didn't try, so I am onboard.

[bors]

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[yoshuawuyts]

bors r+

[bors]

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