update comment text

Author: Yancey0623

paddle/fluid/framework/details/multi_devices_graph_pass.cc

@@ -460,10 +460,14 @@ std::unique_ptr<ir::Graph> MultiDevSSAGraphBuilder::ApplyImpl(
   use_gpu = nccl_ctxs_ != nullptr;
 #endif
 
+  // Insert broadcast operators principle:
+  // 1. Broadcast optimized parameters in Reduce strategy;
+  // 2. No need broadcast optimized parameters in AllReduce strategy because of
+  //    the optimization sub-graph would be run on every GPU;
+  // 3. Allways broadcast received parameters in Distribute Training.
   if ((use_gpu &&
        strategy_.reduce_ == BuildStrategy::ReduceStrategy::kReduce) ||
       is_dist_train) {
-    // allways broadcast receieved parameters for distributed training
     for (size_t dev_id = 0; dev_id < bcast_var_name_set.size(); ++dev_id) {
       auto &to_bcast_set = bcast_var_name_set[dev_id];
       for (auto &bcast_name : to_bcast_set) {