reword the hint::blackbox non-guarantees

People were tripped up by the "precludes", interpreting it that this function
must not ever be used in cryptographic contexts rather than the std lib merely
making zero promises about it being fit-for-purpose.

What remains unchanged is that if someone does try to use it *despite the warnings*
then it is on them to pin their compiler versions and verify the assembly of every
single binary build they do.

library/core/src/hint.rs

@@ -263,7 +263,7 @@ pub fn spin_loop() {
 /// 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*, beyond it behaving as the
 /// identity function. 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
+/// This also means that this function does not offer any guarantees for cryptographic or security
 /// purposes.
 ///
 /// [`std::convert::identity`]: crate::convert::identity