Added upper bound of what vecs and boxes can allocate

src/liballoc/boxed.rs

@@ -2,7 +2,8 @@
 //!
 //! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
-//! drop their contents when they go out of scope.
+//! drop their contents when they go out of scope. Boxes also ensure that they
+//! never allocate more than `isize::MAX` bytes.
 //!
 //! # Examples
 //!

src/liballoc/vec.rs

@@ -4,6 +4,8 @@
 //! Vectors have `O(1)` indexing, amortized `O(1)` push (to the end) and
 //! `O(1)` pop (from the end).
 //!
+//! Vectors ensure they never allocate more than `isize::MAX` bytes.
+//!
 //! # Examples
 //!
 //! You can explicitly create a [`Vec<T>`] with [`new`]: