Add a speaker note about mut function args (#2910)

src/control-flow-basics/solution.md

@@ -3,3 +3,12 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+<details>
+
+- Note that the argument `n` is marked as `mut`, allowing you to change the
+  value of `n` in the function. Like variables, function arguments are immutable
+  by default and you must add `mut` if you want to modify their value. This does
+  not affect how the function is called or how the argument is passed in.
+
+</details>