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.
• 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.

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

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[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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---- library/core/src/hint.rs - hint::assert_unchecked (line 187) stdout ----
error[E0425]: cannot find function `assert_unchecked` in this scope
   --> library/core/src/hint.rs:197:13
    |
197 |    unsafe { assert_unchecked(x >= 0.) };
    |             ^^^^^^^^^^^^^^^^ not found in this scope
    |
help: consider importing this function
    |
188 + use std::hint::assert_unchecked;
    |

error[E0133]: call to unsafe function `classify_not_negative` is unsafe and requires unsafe block
   --> library/core/src/hint.rs:220:9
    |
220 |    dbg!(classify_not_negative(nan));
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
    |
    = note: consult the function's documentation for information on how to avoid undefined behavior

error: aborting due to 2 previous errors