memory-management: add explanatory commentary to solution

src/memory-management/solution.md

@@ -3,3 +3,27 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+- **Consuming Builder:** The methods on `PackageBuilder` take `mut self` and
+  return `Self`. This pattern transfers ownership of the builder into the
+  method, modifies it, and returns ownership back to the caller. This allows
+  method chaining (e.g., `.version(...).authors(...)`).
+- **`impl Into<String>`:** Arguments like `name` and `version` use
+  `impl Into<String>`. This allows the caller to pass anything that can be
+  converted into a `String`, such as a string literal (`&str`) or an owned
+  `String`. It makes the API flexible.
+- **Cloning:** In `as_dependency`, we must `clone()` the name and version. The
+  method takes `&self` (a shared reference), so we cannot move fields out of the
+  package. Since `Dependency` owns its strings, we must create new copies.
+- **Tuple Struct Wrapper:** `PackageBuilder` is a tuple struct wrapping
+  `Package`. We access the inner package via `self.0`. This hides the
+  implementation details of `Package` while it's being built.
+
+<details>
+
+- Discuss why `as_dependency` requires cloning. If it took `self` (by value), it
+  could move the fields, but that would consume the `Package`, preventing
+  further use.
+- The `dependencies` vector owns the `Dependency` structs.
+
+</details>