Clarify contiguous memory array structure of vectors in documentation

Closes #31554.

Contributes to #29380.

src/doc/book/vectors.md

@@ -11,15 +11,21 @@ let v = vec![1, 2, 3, 4, 5]; // v: Vec<i32>
 ```
 
 (Notice that unlike the `println!` macro we’ve used in the past, we use square
-brackets `[]` with `vec!` macro. Rust allows you to use either in either situation,
-this is just convention.)
+brackets `[]` with `vec!` macro. Rust allows you to use either in either
+situation, this is just convention.)
 
 There’s an alternate form of `vec!` for repeating an initial value:
 
 ```rust
 let v = vec![0; 10]; // ten zeroes
 ```
 
+Vectors store their contents as contiguous arrays of `T` on the heap. This means
+that they must be able to know the size of `T` at compile time (that is, how
+many bytes are needed to store a `T`?). The size of some things can't be known
+at compile time. For these you'll have to store a pointer to that thing:
+thankfully, the [`Box`][box] type works perfectly for this.
+
 ## Accessing elements
 
 To get the value at a particular index in the vector, we use `[]`s:
@@ -113,6 +119,7 @@ Vectors have many more useful methods, which you can read about in [their
 API documentation][vec].
 
 [vec]: ../std/vec/index.html
+[box]: ../std/boxed/index.html
 [generic]: generics.html
 [panic]: concurrency.html#panics
 [get]: http://doc.rust-lang.org/std/vec/struct.Vec.html#method.get

src/libcollections/vec.rs

@@ -8,8 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! A growable list type with heap-allocated contents, written `Vec<T>` but
-//! pronounced 'vector.'
+//! A contiguous growable array type with heap-allocated contents, written
+//! `Vec<T>` but pronounced 'vector.'
 //!
 //! Vectors have `O(1)` indexing, amortized `O(1)` push (to the end) and
 //! `O(1)` pop (from the end).
@@ -78,7 +78,7 @@ use borrow::{Cow, IntoCow};
 
 use super::range::RangeArgument;
 
-/// A growable list type, written `Vec<T>` but pronounced 'vector.'
+/// A contiguous growable array type, written `Vec<T>` but pronounced 'vector.'
 ///
 /// # Examples
 ///