Expand docs on std::hint::assert_unchecked

[lukaslueg]

This expands the documentation for std::hint::assert_unchecked(). I tried to rework+expand some rather imprecise language ("simplify things", "UB", "quite unlike anything that would be used in the real world"), reworked the "good usage"-example to not rely on assembly to make its point, and added a "a bool may answer more than you asked for"-counter-example / too-narrow / too-broad. Most importantly for review

• Language was introduced to the effect that UB may time-travel, without getting into the weeds of it; one consequence of UB is that the argument to assert_unchecked is not guaranteed to be evaluated to the extend the programmer might expect from a bool; parts of the paragraph were aligned with the unreachable_unchecked docs.
• On the other hand, assert_unchecked is not just a hint, as the argument needs to be evaluated before entering the function. So the question "How unchecked is unchecked?" can be answered to the effect that the compiler is not allowed to just completely ignore evaluating cond; effects of evaluating cond are guaranteed to be observed.
• Language was introduced to the effect that a panic is guaranteed in debug mode; I'm not sure if ub_checks::assert_unsafe_precondition() should be guaranteed, but if it is, it is good to know for the reader.

I checked that codegen for the examples actually produces what the examples demonstrate.

All tokens used in generating this PR were of purely biological nature.

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

Language was introduced to the effect that UB may time-travel

Technically, UB probably does not today time travel - rust-lang/unsafe-code-guidelines#407... I don't know if structuring the language to suggest otherwise makes sense.

Language was introduced to the effect that a panic is guaranteed in debug mode

I think our current policy is that all ub checks are not guaranteed, so we shouldn't commit to that.

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[rustbot]

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[lukaslueg]

I'll change the PR. Lets wait on rust-lang/reference#2320; once that commits, we can link to it