Indicate that the warning on black_box is a general property of Rust

Author: saethlin

library/core/src/hint.rs

@@ -320,6 +320,9 @@ pub fn spin_loop() {
 /// This also means that this function does not offer any guarantees for cryptographic or security
 /// purposes.
 ///
+/// This limitation is not specific to `black_box`; there is no mechanism in the entire Rust
+/// language that can provide the guarantees required for constant-time cryptography.
+///
 /// </div>
 ///
 /// [`std::convert::identity`]: crate::convert::identity