clarify the safety of core::hint::black_box

[ilka-schulz]

The project uses Rust's core::hint::black_box in constant-time/src/lib.rs (and also in rosenpass/benches/handshake.rs).

The function's documentation (v.s.) says:

As such, it must not be relied upon to control critical program behavior. This immediately precludes any direct use of this function for cryptographic or security purposes.

For a non-cryptographer like me who just skims the source code, this is of course alarming. A comment in the code explaining why this usage of black_box is safe nevertheless (assuming it is) would help the reader a lot.