[vpr][route][lookahead] add comment on router lookahead

Author: amin1377

vpr/src/route/router_lookahead_map.cpp

@@ -330,6 +330,11 @@ std::pair<float, float> MapLookahead::get_expected_delay_and_cong(RRNodeId from_
 
         auto from_ptc = rr_graph.node_ptc_num(from_node);
 
+        /* get_cost_from_src_opin iterates over all routing segments passed to it (the first argument) and returns 
+         * the minimum cost among them. In the following for loop, we iterate over each layer and pass the routing segments 
+         * reachable from the OPIN/SOURCE to segments on that layer. This for loop then calculates and returns 
+         * the minimum cost from the given OPIN/SOURCE to the specified SINK across all layers.
+         */ 
         for (int layer_num = 0; layer_num < device_ctx.grid.get_num_layers(); layer_num++) {
             float this_delay_cost;
             float this_cong_cost;