update comments for HLS on_hls_start

@@ -1295,6 +1295,8 @@ int SrsHls::on_publish(SrsRequest* req)
     }
     
     // notice the source to get the cached sequence header.
+    // when reload to start hls, hls will never get the sequence header in stream,
+    // use the SrsSource.on_hls_start to push the sequence header to HLS.
     if ((ret = source->on_hls_start()) != ERROR_SUCCESS) {
         srs_error("callback source hls start failed. ret=%d", ret);
         return ret;

@@ -653,7 +653,8 @@ int SrsSource::on_hls_start()
 #ifdef SRS_AUTO_HLS
         
     // feed the hls the metadata/sequence header,
-    // when reload to enable the hls.
+    // when reload to start hls, hls will never get the sequence header in stream,
+    // use the SrsSource.on_hls_start to push the sequence header to HLS.
     // TODO: maybe need to decode the metadata?
     if (cache_sh_video && (ret = hls->on_video(cache_sh_video->copy())) != ERROR_SUCCESS) {
         srs_error("hls process video sequence header message failed. ret=%d", ret);