document better

Signed-off-by: Kuat Yessenov <[email protected]>

docs/root/configuration/advanced/well_known_filter_state.rst

@@ -15,14 +15,27 @@ The following list of filter state objects are consumed by Envoy extensions:
    * - ``envoy.tcp_proxy.cluster``
      - | :ref:`TCP proxy <config_network_filters_tcp_proxy>` dynamic cluster name selection
        | on a per-connection basis.
+       | Accepts a cluster name as a constructor.
    * - ``envoy.network.transport_socket.original_dst_address``
      - | :ref:`Original destination cluster <arch_overview_load_balancing_types_original_destination>` dynamic address selection.
+       | Accepts an `IP:PORT` string as a constructor.
        | Fields:
-       | - *ip*: IP address value;
-       | - *port*: port value.
+       | - ``ip``: IP address value as a string;
+       | - ``port``: port value as a number.
    * - ``envoy.upstream.dynamic_host``
      - | :ref:`Dynamic forward proxy <envoy_v3_api_msg_extensions.clusters.dynamic_forward_proxy.v3.ClusterConfig>`
        | upstream host override on a per-connection basis.
+       | Accepts a host string as a constructor.
    * - ``envoy.upstream.dynamic_port``
      - | :ref:`Dynamic forward proxy <envoy_v3_api_msg_extensions.clusters.dynamic_forward_proxy.v3.ClusterConfig>`
        | upstream port override on a per-connection basis.
+       | Accepts a port number string as a constructor.
+
+
+The filter state object fields can be used in the format strings. For example,
+the following format string references the port number in the original
+destination cluster filter state object:
+
+.. code-block:: none
+
+  %FILTER_STATE(envoy.network.transport_socket.original_dst_address:FIELD:port)%