lifetimes: add explanatory commentary to solution

src/lifetimes/solution.md

@@ -3,3 +3,27 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+The solution demonstrates zero-copy parsing, a high-performance pattern made
+safe by Rust's lifetime system:
+
+- **Zero-Copy Parsing:** The `Person` and `PhoneNumber` structs do not own their
+  strings; instead, they borrow them directly from the input byte array. No
+  allocations are performed during parsing.
+- **Enforced Safety:** The lifetime parameter `'a` ties the output structures to
+  the input buffer. The compiler ensures that a `Person` cannot outlive the
+  bytes it was parsed from, preventing use-after-free bugs.
+- **Efficient Slicing:** Using `split_at` allows for efficient traversal of the
+  input buffer by manipulating pointers and lengths without copying data.
+
+<details>
+
+- **Lifetime Propagation:** Note how the lifetime `'a` is propagated through the
+  recursive `parse_message` and `add_field` calls. This creates a chain of
+  borrows that the borrow checker tracks throughout the entire execution.
+- **Trade-offs:** While zero-copy parsing is extremely fast, it requires the
+  original input buffer to remain in memory as long as the parsed data is in
+  use. In some applications, copying into owned `String`s might be preferable to
+  free up the input buffer sooner.
+
+</details>