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.