Split fix into another section for E0749

Author: pickfire

src/librustc_error_codes/error_codes/E0749.md

@@ -11,10 +11,20 @@ trait MyTrait {
 impl !MyTrait for u32 {
     type Foo = i32; // error!
 }
-// impl !MyTrait for u32 {} // fix
 # fn main() {}
 ```
 
 Negative impls are not allowed to have any items. Negative impls declare that a
 trait is **not** implemented (and never will be) and hence there is no need to
 specify the values for trait methods or other items.
+
+One way to fix this is to remove the items in negative impls.
+
+```
+# #![feature(negative_impls)]
+trait MyTrait {
+    type Foo;
+}
+
+impl !MyTrait for u32 {}
+```