Add legacy help text for walletprocesspsbt suggesting to use new RPCs instead.

Author: gwillen

src/wallet/rpcwallet.cpp

@@ -4517,7 +4517,14 @@ UniValue walletprocesspsbt(const JSONRPCRequest& request)
         throw std::runtime_error(
             "walletprocesspsbt \"psbt\" ( sign \"sighashtype\" bip32derivs )\n"
             "\nUpdate a PSBT with input information from our wallet and then sign inputs\n"
-            "that we can sign for.\n"
+            "that we can sign for.\n\n"
+            "NOTE: When working with Confidential Assets transactions, it is necessary to\n"
+            "blind the transaction after filling it in from the wallet and before signing\n"
+            "it. This RPC will fail when working with such transaction. Instead of using\n"
+            "this RPC, use the following sequence:\n"
+            " - walletfillpsbtdata\n"
+            " - blindpsbt\n"
+            " - walletsignpsbt\n"
             + HelpRequiringPassphrase(pwallet) + "\n"
 
             "\nArguments:\n"