clarify correctness of black_box

Author: callumio

Diff for: library/core/src/hint.rs

@@ -216,7 +216,8 @@ pub fn spin_loop() {
 ///
 /// Note however, that `black_box` is only (and can only be) provided on a "best-effort" basis. The
 /// extent to which it can block optimisations may vary depending upon the platform and code-gen
-/// backend used. Programs cannot rely on `black_box` for *correctness* in any way.
+/// backend used. Programs cannot rely on `black_box` for *correctness*, beyond it behaving as the
+/// identity function.
 ///
 /// [`std::convert::identity`]: crate::convert::identity
 ///