Clarify that atomic and regular integers can differ in alignment

The documentation for atomic integers says that they have the "same
in-memory representation" as their underlying integers. This might be
misconstrued as implying that they have the same layout. Therefore,
clarify that atomic integers' alignment is equal to their size.

library/core/src/sync/atomic.rs

@@ -2119,7 +2119,16 @@ macro_rules! atomic_int {
         /// This type has the same in-memory representation as the underlying
         /// integer type, [`
         #[doc = $s_int_type]
-        /// `]. For more about the differences between atomic types and
+        /// `].
+        #[doc = if_not_8_bit! {
+            $int_type,
+            concat!(
+                "However, the alignment of this type is always equal to its ",
+                "size, even on targets where [`", $s_int_type, "`] has a ",
+                "lesser alignment."
+            )
+        }]
+        /// For more about the differences between atomic types and
         /// non-atomic types as well as information about the portability of
         /// this type, please see the [module-level documentation].
         ///