api: add attention to HeaderValueOption.append to avoid removing this field (#23518)

Signed-off-by: wbpcode <wangbaiping@corp.netease.com>

Mirrored from https://github.com/envoyproxy/envoy @ a3ac3e4d6bbebaf3e107f526ae0857020f98d963

envoy/config/core/v3/base.proto

@@ -358,6 +358,11 @@ message HeaderValueOption {
   // existing values. Otherwise it replaces any existing values.
   // This field is deprecated and please use
   // :ref:`append_action <envoy_v3_api_field_config.core.v3.HeaderValueOption.append_action>` as replacement.
+  //
+  // .. note::
+  //   The :ref:`external authorization service <envoy_v3_api_msg_service.auth.v3.CheckResponse>` and
+  //   :ref:`external processor service <envoy_v3_api_msg_service.ext_proc.v3.ProcessingResponse>` have
+  //   default value (``false``) for this field.
   google.protobuf.BoolValue append = 2
       [deprecated = true, (envoy.annotations.deprecated_at_minor_version) = "3.0"];