Clarify upfront that PartialOrd is for strict partial orders

[Muon]

Fixes #140654.

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[Mark-Simulacrum]

I'm going to move this to the libs-api agenda -- maybe we can find someone better suited to review this. It's not clear to me from the description in the issue (#140654) whether this is actually a semantic change or not in what we're promising -- it seems like it is maybe not, based on the diff, but I'm a little confused why we didn't land on this when initially landing the docs here.

[Muon]

I do not believe this a semantic change. That would require the changed sentences to be normative, but that's impossible unless f32 and f64 are not meant to be PartialOrd, since their <= is already not a partial order.

Also note that the docs later clarify that this is indeed for strict partial orders only: https://doc.rust-lang.org/std/cmp/trait.PartialOrd.html#strict-and-non-strict-partial-orders