Explain LazyCell in core::cell overview

Author: workingjubilee

library/core/src/cell.rs

@@ -82,6 +82,21 @@
 //!
 //! The corresponding [`Sync`] version of `OnceCell<T>` is [`OnceLock<T>`].
 //!
+//! ## `LazyCell<T, F>`
+//!
+//! A common pattern with OnceCell is, for a given OnceCell, to use a specific function on every
+//! call to [`OnceCell::get_or_init`] with that cell. This is what is offered by [`LazyCell`],
+//! which pairs cells of `T` with functions of `F`, and calls `F` before it yields `&T`.
+//! This happens implicitly by simply attempting to dereference the LazyCell to get its contents,
+//! so its use is much more transparent with a place which has been initialized by a constant.
+//!
+//! More complicated patterns that don't fit this description can be built on `OnceCell<T>` instead.
+//!
+//! `LazyCell` provides the following methods:
+//!
+//! - [`LazyCell::force`]: initialize the LazyCell and return a reference.
+//!
+//! The corresponding [`Sync`] version of `LazyCell<T, F>` is [`LazyLock<T, F>`].
 //!
 //! # When to choose interior mutability
 //!
@@ -230,6 +245,7 @@
 //! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
 //! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
 //! [`OnceLock<T>`]: ../../std/sync/struct.OnceLock.html
+//! [`LazyLock<T, F>`]: ../../std/sync/struct.LazyLock.html
 //! [`Sync`]: ../../std/marker/trait.Sync.html
 //! [`atomic`]: crate::sync::atomic