borrowing: add explanatory commentary to solution

src/borrowing/solution.md

@@ -3,3 +3,29 @@
 ```rust,editable
 {{#include ../../third_party/rust-on-exercism/health-statistics.rs:solution}}
 ```
+
+The solution explores how structs can capture references and how lifetimes tie
+related data structures together:
+
+- **Lifetimes in Structs:** `HealthReport` contains a reference
+  `patient_name: &'a str`, which necessitates a lifetime parameter `'a`. This
+  guarantees that a report cannot outlive the `User` it was created from.
+- **Mutable Borrows:** `visit_doctor` takes `&mut self` to update the user's
+  stats. Because it returns a `HealthReport` that borrows from `self`, the
+  report's lifetime is tied to the duration of this borrow.
+- **Idiomatic `Option` Handling:** `self.last_blood_pressure.map(...)` is used
+  to concisely compute the change in blood pressure only when a prior value is
+  available.
+
+<details>
+
+- **Lifetime Elision:** The signature
+  `fn visit_doctor(&mut self, ...) -> HealthReport<'_>` uses anonymous
+  lifetimes. The compiler expands this to indicate that the returned report
+  borrows from `self`, effectively making the user inaccessible while the report
+  is in scope.
+- **Borrows and Mutation:** Inside `visit_doctor`, the report must be created
+  _before_ the user's fields are updated, as creating a reference to `self.name`
+  borrows `self`. In Rust, you cannot modify a value while it is borrowed.
+
+</details>