ext_proc: Update the proto comment (envoyproxy#32031)

---------

Signed-off-by: tyxia <[email protected]>

api/envoy/service/ext_proc/v3/external_processor.proto

@@ -91,28 +91,22 @@ message ProcessingRequest {
     // a BodyResponse message, an ImmediateResponse message, or close the stream.
     HttpBody request_body = 4;
 
-    // A chunk of the HTTP request body. Unless ``async_mode`` is ``true``, the server must send back
+    // A chunk of the HTTP response body. Unless ``async_mode`` is ``true``, the server must send back
     // a BodyResponse message or close the stream.
     HttpBody response_body = 5;
 
     // The HTTP trailers for the request path. Unless ``async_mode`` is ``true``, the server
     // must send back a TrailerResponse message or close the stream.
     //
-    // This message is only sent if the trailers processing mode is set to ``SEND``.
-    // If there are no trailers on the original downstream request, then this message
-    // will only be sent (with empty trailers waiting to be populated) if the
-    // processing mode is set before the request headers are sent, such as
-    // in the filter configuration.
+    // This message is only sent if the trailers processing mode is set to ``SEND`` and
+    // the original downstream request has trailers.
     HttpTrailers request_trailers = 6;
 
     // The HTTP trailers for the response path. Unless ``async_mode`` is ``true``, the server
     // must send back a TrailerResponse message or close the stream.
     //
-    // This message is only sent if the trailers processing mode is set to ``SEND``.
-    // If there are no trailers on the original downstream request, then this message
-    // will only be sent (with empty trailers waiting to be populated) if the
-    // processing mode is set before the request headers are sent, such as
-    // in the filter configuration.
+    // This message is only sent if the trailers processing mode is set to ``SEND`` and
+    // the original upstream response has trailers.
     HttpTrailers response_trailers = 7;
   }
 }