Rollup merge of #75702 - GuillaumeGomez:cleanup-e0759, r=pickfire

Clean up E0759 explanation

r? @Dylan-DPC

cc @pickfire

src/librustc_error_codes/error_codes/E0759.md

-A `'static` requirement in a return type involving a trait is not fulfilled.
+Return type involving a trait did not require `'static` lifetime.
 
 Erroneous code examples:
 
 ```compile_fail,E0759
 use std::fmt::Debug;
 
-fn foo(x: &i32) -> impl Debug {
+fn foo(x: &i32) -> impl Debug { // error!
     x
 }
-```
 
-```compile_fail,E0759
-# use std::fmt::Debug;
-fn bar(x: &i32) -> Box<dyn Debug> {
+fn bar(x: &i32) -> Box<dyn Debug> { // error!
     Box::new(x)
 }
 ```
 
-These examples have the same semantics as the following:
+Add `'static` requirement to fix them:
 
 ```compile_fail,E0759
 # use std::fmt::Debug;
-fn foo(x: &i32) -> impl Debug + 'static {
+fn foo(x: &i32) -> impl Debug + 'static { // ok!
     x
 }
-```
 
-```compile_fail,E0759
-# use std::fmt::Debug;
-fn bar(x: &i32) -> Box<dyn Debug + 'static> {
+fn bar(x: &i32) -> Box<dyn Debug + 'static> { // ok!
     Box::new(x)
 }
 ```