Rollup merge of #93356 - pierwill:partialord-headline, r=dtolnay

Edit docs introduction for `std::cmp::PartialOrd`

This makes `PartialOrd` consistent with the other three traits in this module, which all include links to their corresponding mathematical concepts on Wikipedia.

<img width="500" alt="Screen Shot 2022-01-26 at 10 24 23 PM" src="https://user-images.githubusercontent.com/19642016/151291720-decd85ed-cd6e-4be0-84a9-619b98ceb386.png">