Add speaker note about for and array refs (#2665)

src/references/solution.md

@@ -3,3 +3,11 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+<details>
+
+- Note that in `normalize` we were able to do `*item /= mag` to modify each
+  element. This is because we're iterating using a mutable reference to an
+  array, which causes the `for` loop to give mutable references to each element.
+
+</details>