fix comment for parameters end_stream of decodeData/encodeData. (#14620)

Signed-off-by: wangfakang <fakangwang@gmail.com>

include/envoy/http/filter.h

@@ -609,6 +609,7 @@ class StreamDecoderFilter : public StreamFilterBase {
    * Called with a decoded data frame.
    * @param data supplies the decoded data.
    * @param end_stream supplies whether this is the last data frame.
+   * Further note that end_stream is only true if there are no trailers.
    * @return FilterDataStatus determines how filter chain iteration proceeds.
    */
   virtual FilterDataStatus decodeData(Buffer::Instance& data, bool end_stream) PURE;
@@ -843,6 +844,7 @@ class StreamEncoderFilter : public StreamFilterBase {
    * Called with data to be encoded, optionally indicating end of stream.
    * @param data supplies the data to be encoded.
    * @param end_stream supplies whether this is the last data frame.
+   * Further note that end_stream is only true if there are no trailers.
    * @return FilterDataStatus determines how filter chain iteration proceeds.
    */
   virtual FilterDataStatus encodeData(Buffer::Instance& data, bool end_stream) PURE;