message: docs(examples): add more comments to hello server example
diff: #![deny(warnings)]
  +use std::convert::Infallible;
  +
  use hyper::{Body, Request, Response, Server};
  use hyper::service::{make_service_fn, service_fn};
  -async fn hello(_: Request<Body>) -> Result<Response<Body>, hyper::Error> {
  +async fn hello(_: Request<Body>) -> Result<Response<Body>, Infallible> {
  Ok(Response::new(Body::from("Hello World!")))
  }
  @@ -11,17 +13,20 @@ async fn hello(_: Request<Body>) -> Result<Response<Body>, hyper::Error> {
  pub async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
  pretty_env_logger::init();
  - let addr = ([127, 0, 0, 1], 3000).into();
  -
  - let server = Server::bind(&addr)
  - .serve(make_service_fn(|_| {
  + // For every connection, we must make a `Service` to handle all
  + // incoming HTTP requests on said connection.
  + let make_svc = make_service_fn(|_conn| {
  // This is the `Service` that will handle the connection.
  // `service_fn` is a helper to convert a function that
  // returns a Response into a `Service`.
  async {
  - Ok::<_, hyper::Error>(service_fn(hello))
  + Ok::<_, Infallible>(service_fn(hello))
  }
  - }));
  + });
  +
  + let addr = ([127, 0, 0, 1], 3000).into();
  +
  + let server = Server::bind(&addr).serve(make_svc);
  println!("Listening on http://{}", addr);
  