Add link for coherence appendix to Swift forums (#4241)

Co-authored-by: Josh L <josh11b@users.noreply.github.com>
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

docs/design/generics/appendix-coherence.md

@@ -235,3 +235,9 @@ either:
     like
     [dynamic implementation binding](#rejected-variation-dynamic-implementation-binding)
     (though perhaps with a monomorphization cost instead of a runtime cost).
+
+In addition, this can create unsoundness when combined with dynamic downcasts
+and a more complex, less predictable implementation model as
+[discussed in Swift](https://forums.swift.org/t/retroactive-conformances-dynamic-downcast-type-unsoundness/73890).
+This approach would be particularly complex in Carbon due to supporting
+[impl specialization](terminology.md#specialization).