message: Better diagnostics of StarlarkDocs derive
diff: @@ -83,7 +83,16 @@ fn expand_docs_derive(input: DeriveInput) -> syn::Result<proc_macro2::TokenStrea
  );
  quote_spanned! {span=> #frozen::__generated_documentation}
  }
  - false => quote_spanned! {span=> #name::__generated_documentation },
  + false => {
  + if generics.type_params().count() != 0 {
  + return Err(syn::Error::new(
  + span,
  + "If a an item name does not end in `Gen`, it must have no type parameters",
  + ));
  + }
  +
  + quote_spanned! {span=> #name::__generated_documentation }
  + }
  };
  // If we do ConcreteType: Trait<'docs> then we require every instance of ConcreteType
  