Clarify guarantees for `Box` allocation

src/liballoc/alloc.rs

@@ -34,6 +34,9 @@ extern "Rust" {
 /// This type implements the [`Alloc`] trait by forwarding calls
 /// to the allocator registered with the `#[global_allocator]` attribute
 /// if there is one, or the `std` crate’s default.
+///
+/// Note: while this type is unstable, the functionality it provides can be
+/// accessed through the [free functions in `alloc`](index.html#functions).
 #[unstable(feature = "allocator_api", issue = "32838")]
 #[derive(Copy, Clone, Default, Debug)]
 pub struct Global;

src/liballoc/boxed.rs

@@ -4,6 +4,16 @@
 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
 //! drop their contents when they go out of scope.
 //!
+//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
+//! its allocation. It is valid to convert both ways between a [`Box`] and a
+//! raw pointer allocated with the [`Global`] allocator, given that the
+//! [`Layout`] used with the allocator is correct for the type. More precisely,
+//! a `value: *mut T` that has been allocated with the [`Global`] allocator
+//! with `Layout::for_value(&*value)` may be converted into a box using
+//! `Box::<T>::from_raw(value)`. Conversely, the memory backing a `value: *mut
+//! T` obtained from `Box::<T>::into_raw` may be deallocated using the
+//! [`Global`] allocator with `Layout::for_value(&*value)`.
+//!
 //! # Examples
 //!
 //! Move a value from the stack to the heap by creating a [`Box`]: