InsertTokensBefore has non-obvious restriction

I wrote a fixer like this, to mark a class as static:

    private async Task<Document> MarkClassAsStatic(Document document, ClassDeclarationSyntax classDeclaration, CancellationToken c)
    {
        SyntaxToken classDeclarationToken = classDeclaration.Keyword;
        SyntaxToken staticKeywordToken = SyntaxFactory.Token(SyntaxKind.StaticKeyword);

        SyntaxNode root = await document.GetSyntaxRootAsync();
        SyntaxNode newRoot = root.InsertTokensBefore(classDeclarationToken, new[] { staticKeywordToken });

        Document newDocument = document.WithSyntaxRoot(newRoot);
        return newDocument;
    }

The call to InsertTokensBefore fails with the error "System.InvalidOperationException: The item specified is not the element of a list."

It turns out that the first argument to InsertTokensBefore must be an element of a list, and the "class" keyword is not an element of a list. In fact that's why the parameter name is tokenInList. Intellisense shows that, but when I saw the Intellisense for this method I had no idea what the name meant.

And it turns out there's a direct way to do what I want, using the AddModifiers method:

        ClassDeclarationSyntax newClassDeclaration = classDeclaration.AddModifiers(new SyntaxToken[] { staticKeywordToken });

        SyntaxNode root = await document.GetSyntaxRootAsync();
        SyntaxNode newRoot = root.ReplaceNode(classDeclaration, newClassDeclaration);

Still, InsertTokensBefore was so seductive, it was really not obvious to me that it wouldn't work, and the error message didn't lead me towards the right answer.

[gafter]

I'm not sure what would be an appropriate resolution. Do you think the API documentation should be improved?

A more verbose error message on the exception would help. Something to the effect of "you can't pass an arbitrary token as the first argument. The token has to be an element of a SyntaxTokenList."