message: docs: update sponsored transactions docs
  Set the fee in the initial transaction to 0, and set the sponsor nonce
  in the sponsor's transaction.
diff: @@ -146,6 +146,7 @@ const txOptions = {
  contractName: 'contract_name',
  functionName: 'contract_function',
  functionArgs: [bufferCVFromString('foo')],
  + fee: 0,
  senderKey: 'b244296d5907de9864c0b0d51f98a13c52890be0404e83f273144cd5b9960eed01',
  validateWithAbi: true,
  sponsored: true,
  @@ -176,6 +177,7 @@ const sponsorOptions = {
  transaction: deserializedTx,
  sponsorPrivateKey: sponsorKey,
  fee,
  + sponsorNonce: 0,
  };
  const sponsoredTx = await sponsorTransaction(sponsorOptions);
  