update ssc doc

docs/user_guide/source/engines/ssc.rst

@@ -10,7 +10,7 @@ The SSC engine takes the following parameters:
 
 2. ``MpiMode``: Default **TwoSided**. MPI communication modes to use. Besides the default TwoSided mode using two sided MPI communications, MPI_Isend and MPI_Irecv, for data transport, there are four one sided MPI modes: OneSidedFencePush, OneSidedPostPush, OneSidedFencePull, and OneSidedPostPull. Modes with **Push** are based on the push model and use MPI_Put for data transport, while modes with **Pull** are based on the pull model and use MPI_Get. Modes with **Fence** use MPI_Win_fence for synchronization, while modes with **Post** use MPI_Win_start, MPI_Win_complete, MPI_Win_post and MPI_Win_wait.
 
-3. ``Threading``: Default **False**. SSC will use threads to hide the time cost of metadata manipulation and data transfer when this parameter is set to **true**. SSC will check if MPI is initialized with multi-thread enabled, and if not, then SSC will force this parameter to be **false**.
+3. ``Threading``: Default **False**. SSC will use threads to hide the time cost for metadata manipulation and data transfer when this parameter is set to **true**. SSC will check if MPI is initialized with multi-thread enabled, and if not, then SSC will force this parameter to be **false**. Please do NOT enable threading when multiple I/O streams are opened in an application, as it will cause unpredictable errors.
 
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  **Key**                         **Value Format**   **Default** and Examples