add note about breaking change

Author: mark-i-m

src/items/associated-items.md

@@ -134,6 +134,11 @@ let circle_shape = Circle::new();
 let bounding_box = circle_shape.bounding_box();
 ```
 
+> NOTE: Until the 2018 edition, it was possible to declare trait methods with
+> anonymous parameters (e.g. `fn foo(u8)`). This was deprecated and disallowed
+> in the 2018 edition. All parameters must have an argument name. See [RFC
+> 1685](https://github.com/rust-lang/rfcs/pull/1685).
+
 ## Associated Types
 
 *Associated types* are [type aliases] associated with another type. Associated