Add warning about issue with Arbitrum cross-chain utilities (#3626)

(cherry picked from commit 6729fec74a)

contracts/crosschain/arbitrum/CrossChainEnabledArbitrumL2.sol

@@ -17,6 +17,11 @@ import "./LibArbitrumL2.sol";
  * this specialization of {CrossChainEnabled} does not include a constructor.
  *
  * _Available since v4.6._
+ *
+ * WARNING: There is currently a bug in Arbitrum that causes this contract to
+ * fail to detect cross-chain calls when deployed behind a proxy. This will be
+ * fixed when the network is upgraded to Arbitrum Nitro, currently scheduled for
+ * August 31st 2022.
  */
 abstract contract CrossChainEnabledArbitrumL2 is CrossChainEnabled {
     /**