Errors not generated when applied to non-struct item with doc-comments

If you apply the `#[wickerman]` attribute to an item that is not a struct, you should see an error:
```rust
#[wickerman]
enum Foo { // error: this is not a struct
    Bar,
    Baz,
}
```
However, if that item has doc-comments, you won't see the error:
```rust
/// This doesn't generate an error
#[wickerman]
enum Foo {
    Bar,
    Baz,
}
```