message: docs(server): mention http2 support
  mention http2 support in server module documentation.
  Closes
diff: //! It wraps a listener with a [`NewService`](::service), and then should
  //! be executed to start serving requests.
  //!
  +//! [`Server`](Server) accepts connections in both HTTP1 and HTTP2 by default.
  +//!
  //! ## Example
  //!
  //! ```no_run
  @@ -65,7 +67,7 @@ use service::{NewService, Service};
  use self::conn::{Http as Http_, SpawnAll};
  #[cfg(feature = "runtime")] use self::tcp::{AddrIncoming};
  -/// A listening HTTP server.
  +/// A listening HTTP server that accepts connections in both HTTP1 and HTTP2 by default.
  ///
  /// `Server` is a `Future` mapping a bound listener with a set of service
  /// handlers. It is built using the [`Builder`](Builder), and the future
  