Update impl-trait.md (#249)

Adding an extra speaker note why `impl Display` is used very appropriately here and what would go wrong if we tried `T: Display`.

Author: igor-petruk

src/generics/impl-trait.md

@@ -28,4 +28,11 @@ The meaning of `impl Trait` is a bit different in the different positions.
   without naming the type. This can be useful when you don't want to expose the concrete type in a
   public API.
 
+This example is great, because it uses `impl Display` twice. It helps to explain that
+nothing here enforces that it is _the same_ `impl Display` type. If we used a single 
+`T: Display`, it would enforce the constraint that input `T` and return `T` type are the same type.
+It would not work for this particular function, as the type we expect as input is likely not
+what `format!` returns. If we wanted to do the same via `: Display` syntax, we'd need two
+independent generic parameters.
+    
 </details>