methods-and-traits: add explanatory commentary to solution

src/methods-and-traits/solution.md

@@ -3,3 +3,25 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+The solution demonstrates how traits enable polymorphic behavior through
+composition:
+
+- **Trait Implementation:** `VerbosityFilter` implements the `Logger` trait,
+  allowing it to be used anywhere a `Logger` is expected.
+- **Wrapper Pattern:** The `VerbosityFilter` struct wraps an instance of
+  `StderrLogger`. It intercepts the `log` call to apply filtering logic before
+  delegating to the inner logger.
+- **Composition over Inheritance:** Behavior is extended by wrapping one type in
+  another, rather than through a class hierarchy.
+
+<details>
+
+- **Extensibility:** This wrapper pattern (often called a decorator or
+  middleware) allows adding functionality like filtering, buffering, or
+  timestamping without modifying the underlying logger.
+- **Teaser for Generics:** While `VerbosityFilter` is hardcoded to wrap
+  `StderrLogger`, it could be made generic to wrap _any_ type that implements
+  `Logger`. This is covered in the next section.
+
+</details>