ext_authz: fix allowed_upstream_headers_to_append doc (#30919)

allowed_upstream_headers_to_append appends to client request, not response.

Signed-off-by: spacewander <spacewanderlzx@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 86e608693d62d8bec8b1e52035202f7d5ebca7b2

envoy/extensions/filters/http/ext_authz/v3/ext_authz.proto

@@ -334,8 +334,8 @@ message AuthorizationResponse {
   type.matcher.v3.ListStringMatcher allowed_upstream_headers = 1;
 
   // When this :ref:`list <envoy_v3_api_msg_type.matcher.v3.ListStringMatcher>` is set, authorization
-  // response headers that have a correspondent match will be added to the client's response. Note
-  // that coexistent headers will be appended.
+  // response headers that have a correspondent match will be added to the original client request.
+  // Note that coexistent headers will be appended.
   type.matcher.v3.ListStringMatcher allowed_upstream_headers_to_append = 3;
 
   // When this :ref:`list <envoy_v3_api_msg_type.matcher.v3.ListStringMatcher>` is set, authorization