Clarify that trait bounds give access to methods and add example of impl Trait. (#378)

src/generics/trait-bounds.md

@@ -1,19 +1,32 @@
 # Trait Bounds
 
-When working with generics, you often want to limit the types. You can do this
-with `T: Trait` or `impl Trait`:
+When working with generics, you often want to require the types to implement
+some trait, so that you can call this trait's methods.
+
+You can do this with `T: Trait` or `impl Trait`:
 
 ```rust,editable
 fn duplicate<T: Clone>(a: T) -> (T, T) {
     (a.clone(), a.clone())
 }
 
+// Syntactic sugar for:
+//   fn add_42_millions<T: Into<i32>>(x: T) -> i32 {
+fn add_42_millions(x: impl Into<i32>) -> i32 {
+    x.into() + 42_000_000
+}
+
 // struct NotClonable;
 
 fn main() {
     let foo = String::from("foo");
     let pair = duplicate(foo);
     println!("{pair:?}");
+
+    let many = add_42_millions(42_i8);
+    println!("{many}");
+    let many_more = add_42_millions(10_000_000);
+    println!("{many_more}");
 }
 ```