Clarify function pointer variadics also support -unwind

Author: ehuss

src/types/function-pointer.md

@@ -47,6 +47,8 @@ these calling conventions:
 * `win64`
 * `efiapi`
 
+This also includes the corresponding [`-unwind` variants][items.fn.extern.unwind].
+
 An example where `Binop` is defined as a function pointer type:
 
 ```rust