MAC: clarify reconfigCellGroup warning for handover scenario

Document that reconfigCellGroup remaining present is expected during
handover (target DU doesn't send RRC Reconfiguration).

openair2/LAYER2/NR_MAC_gNB/mac_rrc_dl_handler.c

@@ -771,6 +771,10 @@ void ue_context_modification_request(const f1ap_ue_context_mod_req_t *req)
   } else if (req->reconfig_compl) {
     LOG_I(NR_MAC, "DU received confirmation of successful RRC Reconfiguration\n");
     if (UE->reconfigCellGroup) {
+      /** During handover, target DU never sends RRC Reconfiguration (source DU does),
+       * so ack_reconfig is never called on target DU - this warning is expected.
+       * If RRC Reconfiguration was sent via PDSCH, ack_reconfig should have been called when UE ACKed it.
+       * If not, this warning would indicate a bug. */
       LOG_W(NR_MAC, "reconfigCellGroup still present, did we miss ACK for RRCReconfiguration?\n");
       ASN_STRUCT_FREE(asn_DEF_NR_CellGroupConfig, UE->CellGroup);
       UE->CellGroup = UE->reconfigCellGroup;