Rollup merge of #136296 - RalfJung:float-min-max, r=tgross35

float::min/max: mention the non-determinism around signed 0

Turns out this can actually produce different results on different machines [in practice](rust-lang/rust#83984 (comment)); that seems worth documenting. I assume LLVM will happily const-fold these operations so so there could be different results for the same input even on the same machine, depending on whether things get const-folded or not.

`@nikic` I remember there was an LLVM soundness fix regarding scalar evolution for loops that had to recognize certain operations as non-deterministic... it seems to me that pass would also have to avoid predicting the result of `llvm.{min,max}num`, for the same reason?

r? `@tgross35`
Cc `@rust-lang/libs-api`

If this lands we should also make Miri non-deterministic here.

Fixes rust-lang/rust#83984

Author: jhpratt