Rollup merge of rust-lang#75360 - pickfire:patch-4, r=GuillaumeGomez

Add sample fix for E0749

Even though the description is clear but the solution may not be as straightforward.
Adding a suggested fix from documentation side.

r? @GuillaumeGomez

However, this suggestion should be shown in rustc itself for easy fix, the documentation should also reflect on the changes in rustc. Currently,
```
error[E0749]: negative impls cannot have any items
 --> test.rs:6:5
  |
6 |     type Foo = i32; // error!
  |     ^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0749`.
```
rustc should tell the user to remove it.

src/librustc_error_codes/error_codes/E0749.md

@@ -11,9 +11,19 @@ trait MyTrait {
 impl !MyTrait for u32 {
     type Foo = i32; // error!
 }
-# 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 {}
+```