message: clarify OSP_TASK_FINISHED
diff: @@ -2152,8 +2152,9 @@ The following are values which can be synchronized with the application
  OSP_WORLD_RENDERED Wait for the world to be rendered, but not
  post-processing operations (Pixel/Tile/Frame Op)
  OSP_FRAME_FINISHED Wait for all rendering operations to complete
  - OSP_TASK_FINISHED Sentinal value to wait on full completion of any type of
  - task, regardless of operation
  + OSP_TASK_FINISHED Wait on full completion of the task associated with
  + the future. The underlying task may involve one or
  + more of the above synchronization events
  -------------------- --------------------------------------------------------
  : Supported events that can be passed to `ospWait`
  