clean up E0204 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0204.md

@@ -1,21 +1,24 @@
-An attempt to implement the `Copy` trait for a struct failed because one of the
-fields does not implement `Copy`. To fix this, you must implement `Copy` for the
-mentioned field. Note that this may not be possible, as in the example of
+The `Copy` trait was implemented on a type which contains a field that doesn't
+implement the `Copy` trait.
+
+Erroneous code example:
 
 ```compile_fail,E0204
 struct Foo {
-    foo : Vec<u32>,
+    foo: Vec<u32>,
 }
 
-impl Copy for Foo { }
+impl Copy for Foo { } // error!
 ```
 
-This fails because `Vec<T>` does not implement `Copy` for any `T`.
+The `Copy` trait is implemented by default only on primitive types. If your
+type only contains primitive types, you'll be able to implement `Copy` on it.
+Otherwise, it won't be possible.
 
 Here's another example that will fail:
 
 ```compile_fail,E0204
-#[derive(Copy)]
+#[derive(Copy)] // error!
 struct Foo<'a> {
     ty: &'a mut bool,
 }