Rollup merge of rust-lang#104912 - RalfJung:per, r=Mark-Simulacrum

PartialEq: PERs are homogeneous

PartialEq claims that it corresponds to a PER, but that is only a well-defined statement when `Rhs == Self`. There is no standard notion of PER on a relation between two different sets/types. So move this out of the first paragraph and clarify this.

Author: matthiaskrgr