Document array expression with a const.

Author: ehuss

src/expressions/array-expr.md

@@ -22,14 +22,17 @@ such as a [literal](../tokens.md#literals) or a [constant
 item](../items/constant-items.md). `[a; b]` creates an array containing `b`
 copies of the value of `a`. If the expression after the semi-colon has a value
 greater than 1 then this requires that the type of `a` is
-[`Copy`](../special-types-and-traits.md#copy).
+[`Copy`](../special-types-and-traits.md#copy), or `a` must be a path to a
+constant item.
 
 ```rust
 [1, 2, 3, 4];
 ["a", "b", "c", "d"];
 [0; 128];              // array with 128 zeros
 [0u8, 0u8, 0u8, 0u8,];
 [[1, 0, 0], [0, 1, 0], [0, 0, 1]]; // 2D array
+const EMPTY: Vec<i32> = Vec::new();
+[EMPTY; 2];
 ```
 
 ### Array expression attributes