Mention that structs can contain &mut Ts

Fixes #30254

src/doc/book/structs.md

@@ -88,6 +88,35 @@ fn main() {
 }
 ```
 
+Your structure can still contain `&mut` pointers, which will let
+you do some kinds of mutation:
+
+```rust
+struct Point {
+    x: i32,
+    y: i32,
+}
+
+struct PointRef<'a> {
+    x: &'a mut i32,
+    y: &'a mut i32,
+}
+
+fn main() {
+    let mut point = Point { x: 0, y: 0 };
+
+    {
+        let r = PointRef { x: &mut point.x, y: &mut point.y };
+
+        *r.x = 5;
+        *r.y = 6;
+    }
+
+    assert_eq!(5, point.x);
+    assert_eq!(6, point.y);
+}
+```
+
 # Update syntax
 
 A `struct` can include `..` to indicate that you want to use a copy of some