float types: document NaN bit pattern guarantees

[RalfJung]

Part of https://github.com/rust-lang/rust/issues/128288: document the guarantees we make for NaN bit patterns.

Cc @tgross35

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[tgross35]

Cc @beetrees (more of a float expert than me)

[beetrees]

Would it be worth also updating the docs for copysign to say something about what happens when the sign argument is a NaN (similar to the addition to is_sign_{positive,negative} docs)? Currently the the copysign docs only discuss what happens when self is a NaN. The link in the copysign docs could also be updated to point to the new NaN bit patterns section.

[RalfJung]

Fair points, I think I took care of all of them. :)

[thomcc]

Looks fine to me. This is part of an approved RFC and is doc-only so I don't think it needs any FCP or anything.

@bors r+ rollup

[bors]

:pushpin: Commit 0c7d6c45e6b664fa4875741e78d185845d374f07 has been approved by thomcc

It is now in the queue for this repository.