Clarify atomic bit validity

The previous definition used the phrase "representation", which is ambiguous given the current state of memory model nomenclature in Rust. The new wording clarifies that size and bit validity are guaranteed to match the corresponding native integer type.

library/core/src/sync/atomic.rs

@@ -2121,7 +2121,7 @@ macro_rules! atomic_int {
      $int_type:ident $atomic_type:ident) => {
         /// An integer type which can be safely shared between threads.
         ///
-        /// This type has the same in-memory representation as the underlying
+        /// This type has the same in-memory size and bit validity as the underlying
         /// integer type, [`
         #[doc = $s_int_type]
         /// `].