Add note to LabelTransaction TXID field

The description for TXID in Label Transaction should include a note
that, in the case of GRPC, the TXID needs to be passed in little-endian
(reverse) byte order. A user passing the bytes in big-endian order is
met with an error message stating that the transaction is not found in
the wallet and no other information with which to troubleshoot.

lnrpc/walletrpc/walletkit.proto

@@ -982,7 +982,8 @@ message ListSweepsResponse {
 }
 
 message LabelTransactionRequest {
-    // The txid of the transaction to label.
+    // The txid of the transaction to label. Note: When using gRPC, the bytes
+    // must be in little-endian (reverse) order.
     bytes txid = 1;
 
     // The label to add to the transaction, limited to 500 characters.