Add SAFETY comment for atomic example

Author: foeb

src/libcore/sync/atomic.rs

@@ -1989,7 +1989,9 @@ extern {
 }
 
 let mut atomic = ", stringify!($atomic_type), "::new(1);
-unsafe {
+",
+// SAFETY: Safe as long as `my_atomic_op` is atomic.
+"unsafe {
     my_atomic_op(atomic.as_mut_ptr());
 }
 # }