Enhance documentation for Swashbuckle and NSwag

We recently did some gap analysis between what you get naively from a swagger framework, and what a 'canonical' swagger document looks like. Many of the gaps are features that are already included by 3rd party swagger frameworks like NSwag and Swashbuckle.

We'd like to add additional docs coverage of how to configure all of the items here: https://github.com/aspnet/Mvc/issues/6877