docs: Fixing ECDS documentation (#23636)

Signed-off-by: Yanjun Xiang <yanjunxiang@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 1f176807e49558fd18a41a75ef4d81f8de11eff2

envoy/service/extension/v3/config_discovery.proto

@@ -18,6 +18,25 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 
 // [#protodoc-title: Extension config discovery service (ECDS)]
 
+// A service that supports dynamic configuration updates for a specific filter.
+// Currently, ECDS is supported for HTTP filters and Listener filters. Please check
+// :ref:`Extension Config Discovery Service (ECDS) API <config_overview_extension_discovery>`.
+// The overall extension config discovery service works as follows:
+//
+// 1. A filter (:ref:`Listener <envoy_v3_api_field_config.listener.v3.ListenerFilter.config_discovery>`
+//    or :ref:`HTTP <envoy_v3_api_field_extensions.filters.network.http_connection_manager.v3.HttpFilter.config_discovery>`)
+//    contains a :ref:`config_discovery <envoy_v3_api_msg_config.core.v3.ExtensionConfigSource>` configuration. This configuration
+//    includes a :ref:`config_source <envoy_v3_api_field_config.core.v3.ExtensionConfigSource.config_source>`,
+//    from which the filter configuration will be fetched.
+// 2. The client then registers for a resource using the filter name as the resource_name.
+// 3. The xDS server sends back the filter's configuration.
+// 4. The client stores the configuration that will be used in the next instantiation of the filter chain,
+//    i.e., for the next requests. Whenever an updated filter configuration arrives, it will be taken into
+//    account in the following instantiation of the filter chain.
+//
+// Note: Filters that are configured using ECDS are warmed. For more details see
+// :ref:`ExtensionConfigSource <envoy_v3_api_msg_config.core.v3.ExtensionConfigSource>`.
+
 // Return extension configurations.
 service ExtensionConfigDiscoveryService {
   option (envoy.annotations.resource).type = "envoy.config.core.v3.TypedExtensionConfig";