generics: add explanatory commentary to solution

src/generics/solution.md

@@ -3,3 +3,24 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+- **Generic Function:** `min` is a generic function that takes two arguments of
+  type `T`.
+- **Trait Bounds:** The syntax `<T: Ord>` specifies that `T` must implement the
+  `Ord` trait. This is necessary because not all types can be compared (e.g.,
+  floating-point numbers in Rust only implement `PartialOrd` due to `NaN`).
+- **`Ord` Trait:** The `Ord` trait provides the `cmp` method, which compares two
+  values and returns an `Ordering`.
+- **`Ordering` Enum:** The result of `cmp` is an enum with variants `Less`,
+  `Equal`, and `Greater`. We use `match` to handle these cases.
+
+<details>
+
+- Mention that for floating point numbers, `f64` does not implement `Ord`, so
+  this function would not work for them. This is a deliberate design choice in
+  Rust to handle `NaN` correctly (NaN != NaN). To handle floats, one would
+  typically use `PartialOrd` or a wrapper type.
+- Alternatively, `l <= r` works if we use `T: PartialOrd`. However, `Ord` is
+  stricter and guarantees a total order, which `cmp` relies on.
+
+</details>