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.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",