message: [doc] clarify getdata limit after
  GETDATA is limited to blocks and transactions now and can't be used for other non-block data
diff: @@ -4726,7 +4726,7 @@ bool PeerManager::SendMessages(CNode* pto)
  }
  //
  - // Message: getdata (non-blocks)
  + // Message: getdata (transactions)
  //
  std::vector<std::pair<NodeId, GenTxid>> expired;
  auto requestable = m_txrequest.GetRequestable(pto->GetId(), current_time, &expired);
  