message: clarify comment regarding access of chain_id_type() constructor
diff: @@ -37,12 +37,12 @@ namespace chain {
  private:
  chain_id_type() = default;
  + // Some exceptions are unfortunately necessary:
  template<typename T>
  friend T fc::variant::as()const;
  friend class eosio::chain_apis::read_only;
  - // Eventually net_plugin should be updated to not default construct chain_id. Then these two lines can be removed.
  friend class eosio::net_plugin_impl;
  friend struct eosio::handshake_message;
  };
  