Include field rustdocs in generated code

[dcreager]

If you add rustdocs to a tracked or interned struct:

/// A description of the struct itself
#[salsa::tracked]
struct Yay {
    /// An exquisitely useful description of this field
    field: u32,
}

then the comment for the struct itself is carried over to the generated code, but the comments for each field are not. It would be nice if those were carried over to the accessor functions that are generated for the field.

[puuuuh]

Looks like duplicate of #728 I'll try to implement this