message: doc(client): document that `http2_only` needs Prior Knowledge or ALPN
  configured.
  Closes
diff: @@ -714,7 +714,7 @@ impl<B> ClientError<B> {
  /// A marker to identify what version a pooled connection is.
  #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
  enum Ver {
  - Http1,
  + Auto,
  Http2,
  }
  @@ -792,7 +792,7 @@ impl Default for Builder {
  client_config: Config {
  retry_canceled_requests: true,
  set_host: true,
  - ver: Ver::Http1,
  + ver: Ver::Auto,
  },
  conn_builder: conn::Builder::new(),
  pool_config: pool::Config {
  @@ -864,6 +864,11 @@ impl Builder {
  /// Set whether the connection **must** use HTTP/2.
  ///
  + /// The destination must either allow HTTP2 Prior Knowledge, or the
  + /// `Connect` should be configured to do use ALPN to upgrade to `h2`
  + /// as part of the connection process. This will not make the `Client`
  + /// utilize ALPN by itself.
  + ///
  /// Note that setting this to true prevents HTTP/1 from being allowed.
  ///
  /// Default is false.
  @@ -871,7 +876,7 @@ impl Builder {
  self.client_config.ver = if val {
  Ver::Http2
  } else {
  - Ver::Http1
  + Ver::Auto
  };
  self
  }
  