borrowing: 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/borrowing/solution.md

@@ -3,3 +3,25 @@
 ```rust,editable
 {{#include ../../third_party/rust-on-exercism/health-statistics.rs:solution}}
 ```
+
+- **Lifetimes in Structs:** `HealthReport` has a lifetime parameter `'a` because
+  it contains a reference `patient_name: &'a str`. This ensures that the report
+  cannot outlive the `User` it refers to.
+- **Mutable Reference (`&mut self`):** `visit_doctor` modifies the `User`
+  struct, so it must take `&mut self`.
+- **Lifetime Elision:** The return type `HealthReport<'_>` indicates that the
+  output lifetime is tied to the input lifetime of `self`. Explicitly, this
+  would be `fn visit_doctor<'a>(&'a mut self, ...) -> HealthReport<'a>`.
+- **Option combinators:** We use `self.last_blood_pressure.map(...)` to
+  convenienty calculate the blood pressure change if the previous measurement
+  exists.
+
+<details>
+
+- Explain that `HealthReport` borrows from `User`. While `report` exists, `User`
+  is borrowed (mutably, because it came from `visit_doctor`), so we cannot use
+  `User` for anything else until `report` is dropped.
+- Note the cast to `i32` for blood pressure calculation to allow for negative
+  changes.
+
+</details>