clarify how Rust atomics correspond to C++ atomics

Author: RalfJung

library/core/src/sync/atomic.rs

@@ -4,6 +4,12 @@
 //! threads, and are the building blocks of other concurrent
 //! types.
 //!
+//! Rust atomics follow the same rules as [C++20 atomics][cpp], specifically `atomic_ref`.
+//! Basically, creating a *shared reference* to one of the Rust atomic types corresponds to creating
+//! an `atomic_ref` in C++; the `atomic_ref` is destroyed when the lifetime of the shared reference
+//! ends. (A Rust atomic type that is exclusively owned or behind a mutable reference does *not*
+//! correspond to an "atomic object" in C++, since it can be accessed via non-atomic operations.)
+//!
 //! This module defines atomic versions of a select number of primitive
 //! types, including [`AtomicBool`], [`AtomicIsize`], [`AtomicUsize`],
 //! [`AtomicI8`], [`AtomicU16`], etc.
@@ -14,6 +20,7 @@
 //! the memory barrier for that operation. These orderings are the
 //! same as the [C++20 atomic orderings][1]. For more information see the [nomicon][2].
 //!
+//! [cpp]: https://en.cppreference.com/w/cpp/atomic
 //! [1]: https://en.cppreference.com/w/cpp/atomic/memory_order
 //! [2]: ../../../nomicon/atomics.html
 //!