Add additional explain for quiz3 solution

solutions/quizzes/quiz3.rs

@@ -10,6 +10,14 @@
 use std::fmt::Display;
 
 // Make the struct generic over `T`.
+//
+// Note: We could also add the Display trait bound here at the struct level, i.e.,
+// `struct ReportCard<T: Display> { ... }`. That would enforce that every `ReportCard` instance
+// must have a grade that is displayable. However, by not placing the bound here, we allow
+// the creation of `ReportCard` instances with non-displayable grades (but then we cannot use the `print` method).
+// Since the main purpose of this struct is to be printable, it would also be reasonable to add the bound at the struct level.
+// In this solution, we choose to put the bound only in the `impl` block to show that we can conditionally
+// implement methods only when the type satisfies certain traits. This offers more flexibility in some advanced scenarios.
 struct ReportCard<T> {
     //           ^^^
     grade: T,
@@ -19,6 +27,10 @@ struct ReportCard<T> {
 }
 
 // To be able to print the grade, it has to implement the `Display` trait.
+//
+// Note: We implement the `print` method only for `ReportCard<T>` where `T` implements `Display`.
+// This means that if we have a `ReportCard` with a grade that doesn't implement `Display`,
+// we can still create the instance, but we cannot call the `print` method on it.
 impl<T: Display> ReportCard<T> {
     //  ^^^^^^^ require that `T` implements `Display`.
     fn print(&self) -> String {