Explicate what "Rc" and "Arc" stand for.

src/liballoc/arc.rs

@@ -42,7 +42,8 @@
 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 
-/// A thread-safe reference-counting pointer.
+/// A thread-safe reference-counting pointer. "Arc" stands for "Atomically
+/// Reference Counted".
 ///
 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces

src/liballoc/rc.rs

@@ -10,7 +10,8 @@
 
 #![allow(deprecated)]
 
-//! Single-threaded reference-counting pointers.
+//! Single-threaded reference-counting pointers. "Rc" stands for "Reference
+//! Counted".
 //!
 //! The type [`Rc<T>`][`Rc`] provides shared ownership of a value of type `T`,
 //! allocated in the heap. Invoking [`clone`][clone] on [`Rc`] produces a new