Update refund docs on IERC7984Receiver (#257)

* Update refund docs on `IERC7984Receiver`

* Update contracts/interfaces/IERC7984Receiver.sol

Author: arr00

contracts/interfaces/IERC7984Receiver.sol

@@ -8,7 +8,9 @@ import {ebool, euint64} from "@fhevm/solidity/lib/FHE.sol";
 interface IERC7984Receiver {
     /**
      * @dev Called upon receiving a confidential token transfer. Returns an encrypted boolean indicating success
-     * of the callback. If false is returned, the transfer must be reversed.
+     * of the callback. If false is returned, the token contract will attempt to refund the transfer.
+     *
+     * WARNING: Do not manually refund the transfer AND return false, as this can lead to double refunds.
      */
     function onConfidentialTransferReceived(
         address operator,