Add match order example.

Author: ehuss

src/expressions/match-expr.md

@@ -74,6 +74,14 @@ let message = match x {
 };
 
 assert_eq!(message, "a few");
+
+// Demonstration of pattern match order.
+struct S(i32, i32);
+
+match S(1, 2) {
+    S(z @ 1, _) | S(_, z @ 2) => assert_eq!(z, 1),
+    _ => panic!(),
+}
 ```
 
 > Note: The `2..=9` is a [Range Pattern], not a [Range Expression]. Thus, only