Add comment to the FDD conf file

ci-scripts/conf_files/gnb-du.sa.band1.52prb.usrpb210.conf

@@ -190,6 +190,8 @@ RUs = (
        local_rf       = "yes"
          nb_tx          = 1
          nb_rx          = 1
+         # The higher att for TX than RX is because we use a circulator (as for TDD),
+         # while we should use a duplexer in the FDD case. However, it uses the same setup.
          att_tx         = 20
          att_rx         = 0;
          bands          = [1];