references: add explanatory commentary to solution

This commentary, written by Gemini, focuses on aspects of the solution that differ from the baseline languages (C/Java/Python), highlighting Rust-specific idioms and concepts.

Author: mgeisler

src/references/solution.md

@@ -4,6 +4,18 @@
 {{#include exercise.rs:solution}}
 ```
 
+This solution highlights the difference between shared and mutable references:
+
+- **Shared References (`&`):** `magnitude` needs to read the vector components
+  but not modify them, so it takes a shared reference (`&[f64; 3]`).
+- **Mutable References (`&mut`):** `normalize` modifies the vector in place, so
+  it requires an exclusive mutable reference (`&mut [f64; 3]`).
+- **Dereferencing:** In the `normalize` loop, `item` is a `&mut f64`. To modify
+  the actual value, we must dereference it using `*item`.
+- **Iteration:** When we iterate over a reference to an array (like `vector`),
+  the iterator yields references to the elements. In `magnitude`, `coord` is
+  `&f64`. In `normalize`, `item` is `&mut f64`.
+
 <details>
 
 - Note that in `normalize` we were able to do `*item /= mag` to modify each