feat: allow codegen-ing enum variant docs

It is currently not possible to codegen enum variant docs.

This PR solves this issue.

Since it is likely fairly closely related do documenting named variants, I also added the methods for this instead of moving this to another PR

enum Foo {
    /// Documentation for Bar variant
    Bar {
        /// Documentation for the named bo field
        bo: bool,
    }
    ,
    /// Documentation for Baz variant
    /// With multiple lines
    Baz,
    /// Documentation for Bazinga variant
    Bazinga(String),
}

[!NOTE]

I Know that the comma after the named field of the enum variant is a formatting bug.

It is unrelated to this code, so I left it in.