Add code example for wrapping_neg method for signed integers

Author: GuillaumeGomez

core/src/num/int_macros.rs

@@ -2101,6 +2101,7 @@ macro_rules! int_impl {
         ///
         /// ```
         #[doc = concat!("assert_eq!(100", stringify!($SelfT), ".wrapping_neg(), -100);")]
+        #[doc = concat!("assert_eq!((-100", stringify!($SelfT), ").wrapping_neg(), 100);")]
         #[doc = concat!("assert_eq!(", stringify!($SelfT), "::MIN.wrapping_neg(), ", stringify!($SelfT), "::MIN);")]
         /// ```
         #[stable(feature = "num_wrapping", since = "1.2.0")]