fixup! docs: Correct terminology in std::cmp

In `std::cmp` module documentation, avoid claiming that `PartialEq` corresponds
to partial equivalence relations (except when `Rhs` is `Self`).

Author: JanBeh

library/core/src/cmp.rs

@@ -3,11 +3,11 @@
 //! This module contains various tools for comparing and ordering values. In
 //! summary:
 //!
-//! * [`Eq`] and [`PartialEq`] correspond to equivalence relations and partial
-//!   equivalence relations, respectively. Implementing these traits for types
-//!   provides the `==` and `!=` operators for those types.
-//!   Implementing `Eq` (in addition to `PartialEq`) indicates that `a == a` is
-//!   `true` for every `a` (according to an equivalence relation).
+//! * [`PartialEq<Rhs>`] overloads the `==` and `!=` operators. In cases where
+//!   `Rhs` (the right hand side's type) is `Self`, this corresponds to a
+//!   partial equivalence relation.
+//! * [`Eq`] indicates that the overloaded `==` operator corresponds to an
+//!   equivalence relation.
 //! * [`Ord`] and [`PartialOrd`] are traits that allow you to define total and
 //!   partial orderings between values, respectively. Implementing them overloads
 //!   the `<`, `<=`, `>`, and `>=` operators.