Document expectations about NameResolver SocketAddresses

Currently, a name resolver returns a collection of `SocketAddress` wrapped up in `EquivalentAddressGroup`s. In the most common case InetSocketAddress will be used. Currently there are no expectations about the data in the address, but I believe we need to clarify this.

This came up in working with HTTP Proxies. In case a Proxy is used, Netty can derive the the initial CONNECT message from the InetSocketAddress hostname. This may be present or may not, but the behavior of this may change based on what the name resolver returns.

I don't know what to say here, other than it may be absent, but I wanted to open up a discussion because it recently came up.

まず NameResolver と EquivalentAddressGroup の API ドキュメントを確認し、特に HTTP プロキシで InetSocketAddress のホスト名がどのように使用されるかを確認してください。期待される SocketAddress データを、ホスト名が存在しない場合があるかどうかも含めて文書化し、その結果生じる動作を明確にしてください。

索引モデルが issue の本文から書いたものです。