Add note about anon params

src/introduction.md

@@ -79,6 +79,12 @@ information. These conventions are documented here.
 
   An *example term* is an example of a term beind defined.
 
+* Differences in the language by which edition the crate is compiled under are
+  in a blockquote that start with the words "Edition Differences:" in **bold**.
+
+  > **Edition Differences**: In the 2015 edition, this syntax is valid that is
+  > disallowed as of the 2018 edition.
+
 * Notes that contain useful information about the state of the book or point out
   useful, but mostly out of scope, information are in blockquotes that start
   with the word "Note:" in **bold**.

src/items/associated-items.md

@@ -134,6 +134,10 @@ let circle_shape = Circle::new();
 let bounding_box = circle_shape.bounding_box();
 ```
 
+> **Edition Differences**: In the 2015 edition, it is possible to declare trait
+> methods with anonymous parameters (e.g. `fn foo(u8)`). This is deprecated and
+> an error as of the 2018 edition. All parameters must have an argument name.
+
 ## Associated Types
 
 *Associated types* are [type aliases] associated with another type. Associated