improves documentation in routine compute_src_cell_centroids

src/core/interp_method_conserv.c

@@ -783,6 +783,11 @@ static yac_coordinate_pointer compute_src_cell_centroids(
         src_cell_centroid[2] += super_cell_area * super_cell_barycenter[2];
       }
     } else {
+
+      // In case an unmasked source cell is not covered by any unmasked target
+      // cell (curr_num_super_cells == 0) its centroid is only used to compute
+      // the gradiants of its neighbour cells. In that case a rough estimate
+      // of its centroid (computed here) is sufficient.
       yac_const_basic_grid_data_get_grid_cell(
         src_basic_grid_data, curr_src_cell, &src_grid_cell);
       for (size_t j = 0; j < src_grid_cell.num_corners; ++j) {