`PartialOrd` does not mention upfront that it omits reflexivity

Location

https://doc.rust-lang.org/std/cmp/trait.PartialOrd.html

Summary

A partial order is a reflexive, antisymmetric and transitive relation. The docs for PartialOrd call it a "trait for types that form a partial order". However, we only require transitivity (explicitly) and antisymmetry (by implication from duality, conditions 1-4 in the docs for PartialOrd, and the symmetry of == from the docs of PartialEq). We cannot require reflexivity because f32 and f64 are PartialOrd but x == x is true for them iff x is not NaN, which is mentioned later, but it's a bit buried. The docs should make clear upfront that reflexivity is not required.

Start with the PartialOrd trait documentation at the linked std::cmp page, especially its opening description and the later section on strict and non-strict partial orders. Update the upfront explanation so it clearly states that reflexivity is not required, then verify that the rendered documentation accurately communicates the relationship to types such as f32 and f64.