vpr: router_lookahead_map: comment the overall strategy on min_opin_distance_cost_map

Author: amin1377

vpr/src/route/router_lookahead_map.cpp

@@ -813,6 +813,13 @@ static void min_chann_global_cost_map(vtr::NdMatrix<util::Cost_Entry, 4>& distan
 }
 
 static void min_opin_distance_cost_map(const util::t_src_opin_delays& src_opin_delays, vtr::NdMatrix<util::Cost_Entry, 5>& distance_min_cost) {
+    /**
+     * This function calculates and stores the minimum cost to reach a point on layer `n_sink`, which is `dx` and `dy` further from the current point
+     * on layer `n_source` and is located on physical tile type `t`. To compute this cost, the function iterates over all output pins of tile `t`,
+     * and for each pin, iterates over all segment types accessible by it. It then determines and stores the minimum cost to the destination point.
+     * "src_opin_delays" stores the routing segments accessible by each OPIN of each physical type on each layer. After getting the accessible segment types,
+     * "get_wire_cost_entry" is called to get the cost from that segment type to the destination point.
+     */
     int num_tile_types = g_vpr_ctx.device().physical_tile_types.size();
     int num_layers = g_vpr_ctx.device().grid.get_num_layers();
     int width = (int)g_vpr_ctx.device().grid.width();