iterators: add explanatory commentary to solution

src/iterators/solution.md

@@ -3,3 +3,25 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+The solution demonstrates the power of iterator method chaining to express
+complex logic concisely:
+
+- **Infinite Iteration:** `cycle()` creates an iterator that repeats the input
+  indinitely, elegantly handling the wraparound requirement without modulo
+  arithmetic.
+- **Iterator Composition:** `zip()` combines the original and offset iterators.
+  It naturally terminates when the shorter (finite) iterator is exhausted.
+- **Functional Transformation:** `map()` applies the subtraction, and
+  `collect()` performs the final transformation back into a `Vec`.
+
+<details>
+
+- **Laziness:** Iterator chains are lazy. No work is done until a terminal
+  operation like `collect()` is called. The compiler is often able to optimize
+  these chains into a single tight loop that is as efficient as imperative code.
+- **Reference Handling:** Note that `values.iter()` yields references (`&i32`).
+  In the `map` closure, we use the dereference operator (`*`) to access the
+  underlying values for arithmetic.
+
+</details>