Link between nested type siblings

[slashmo]

Given the following types:

enum Namespace {
  struct A {
    /// - Returns: A `B`.
    func b() -> B { B() }
  }
  struct B {
     /// - Returns: An `A`.
     func a() -> A { A() }
  }
}

It would be great if the generated HTML included links between the two nested types.