Rollup merge of #66566 - robamler:issue-66476, r=rkruppe
Rollup merge of #66566 - robamler:issue-66476, r=rkruppe

Document pitfall with `impl PartialEq<B> for A`

Fixes #66476 by turning the violating example into an explicit
counterexample.

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust