Add missing xml comments

:santa: 
I followed the comments from elsewhere in the class, which also match the [WithCookie](https://github.com/NancyFx/Nancy/blob/4cc05c63cbcf052342131d1beb43e4d9e1bf4aa3/src/Nancy/ResponseExtensions.cs#L61) extension method, but it looks like you're right and the comments are wrong.