Document **all** safety requirements or remove undocumented unsafe functions from public API

[Kixunil]

Several unsafe functions have this in their Safety documentation:

This list is not exhaustive, and any violation may lead to Undefined Behavior.

Which is essentially same as saying: "if you don't obey secret rules this is UB" and since the code can never guarantee obeying secret safety rules it's equivalent to core::hint::unreachable_unchecked but without improved performance.

In other words those functions are totally useless.

Thus I propose for each function to either document all the rules (and make really, really sure they are correct) or just remove it from API if the rules can not be guaranteed.

[tcharding]

We also need to be super careful in future reviewing patches to functions with safter docs that the docs don't go stale. I was a bit worried about this when I wrote docs that are based on line-by-line parts of the function, they are begging to go stale.

[apoelstra]

In other words those functions are totally useless.

Well, in fairness, it's more like "they're useless unless you read all the source, including the C source, and you know all the UB rules for both languages and you're logically omniscient" ;).

But I agree, we absolutely should fix this.