Rollup merge of #75451 - GuillaumeGomez:cleanup-e0751, r=pickfire

Clean up E0751 explanation

r? @Dylan-DPC

cc @pickfire

Author: JohnTitor

src/librustc_error_codes/error_codes/E0751.md

@@ -5,8 +5,8 @@ Erroneous code example:
 ```compile_fail,E0751
 trait MyTrait {}
 impl MyTrait for i32 { }
-impl !MyTrait for i32 { }
+impl !MyTrait for i32 { } // error!
 ```
 
-Negative implementations are a promise that the trait will never be
-implemented for the given types.
+Negative implementations are a promise that the trait will never be implemented
+for the given types. Therefore, both cannot exists at the same time.