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.

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