message: added DoubleBuffer engine parameter in dataman doc
diff: @@ -32,6 +32,10 @@ The DataMan engine takes the following parameters:
  A number >1 will cause the writer to wait for more readers, and a value of 0 will allow the writer to proceed without any readers present.
  This value is interpreted by DataMan Writer engines only.
  +5. ``DoubleBuffer``: Default **true** for reader, **false** for writer. Whether to use double buffer for caching send and receive operations.
  + Enabling double buffer will cause extra overhead for managing threads and buffer queues, but will improve the continuity of data steps for the reader, for the pub/sub mode.
  + Advice for generic uses cases is to keep the default values, true for reader and false for writer.
  +
  =============================== ================== ================================================
  **Key** **Value Format** **Default** and Examples
  =============================== ================== ================================================
  @@ -39,6 +43,7 @@ The DataMan engine takes the following parameters:
  Port integer **50001**, 22000, 33000
  Timeout integer **5**, 10, 30
  RendezvousReaderCount integer **1**, 0, 3
  + DoubleBuffer bool **true** for reader, **false** for writer
  =============================== ================== ================================================
  