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.pb.go

@@ -3023,7 +3023,8 @@ type LabelTransactionRequest struct {
 	sizeCache     protoimpl.SizeCache
 	unknownFields protoimpl.UnknownFields
 
-	// 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.
 	Txid []byte `protobuf:"bytes,1,opt,name=txid,proto3" json:"txid,omitempty"`
 	// The label to add to the transaction, limited to 500 characters.
 	Label string `protobuf:"bytes,2,opt,name=label,proto3" json:"label,omitempty"`

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.

lnrpc/walletrpc/walletkit.swagger.json

@@ -1525,7 +1525,7 @@
         "txid": {
           "type": "string",
           "format": "byte",
-          "description": "The txid of the transaction to label."
+          "description": "The txid of the transaction to label. Note: When using gRPC, the bytes\nmust be in little-endian (reverse) order."
         },
         "label": {
           "type": "string",