Disallow Placing Doc-Comments Directly on Variant Fields

[InsertCreativityHere]

Slice currently allows you two different ways to place a doc-comment on the fields of a variant:

enum MyEnum {
    MyVariant(
         /// I can place a doc-comment directly on my field.
        name: string
    )
}

and

enum MyEnum {
    /// @param name: I can place a doc-comment like this too.
    MyVariant(name: string)
}

I propose we drop support for one of these to simplify the syntax, and the information we need to transmit to code-generator plugins, and of the two, I think we should drop support for the first syntax (placing comments directly on fields).

[bernardnormier]

Is #684 already implemented?

[InsertCreativityHere]

Yes, it's implemented, but the issue hasn't been closed because we still need to add tests for it.
And there's some holes found in the validation. See #684 (comment)