[feature/upp_2d_decomp] Update numx documentation to note that it is …

…used for offline post 2D decomposition

docs/UsersGuide/source/CustomizingTheWorkflow/ConfigWorkflow.rst

@@ -1303,7 +1303,7 @@ Set parameters for customizing the :term:`UPP`.
    The file that lists grib2 fields to be extracted for testbed files. An empty string means no need to generate testbed files.
 
 ``NUMX``: (Default: 1)
-   The number of ``i`` regions in a 2D decomposition. Each ``i`` row is distributed to ``NUMX`` ranks. Set ``NUMX`` to a value greater than 1 to enable 2D decomposition.
+   The number of ``i`` regions in a 2D decomposition. Each ``i`` row is distributed to ``NUMX`` ranks. Used for offline post 2D decomposition. Set ``NUMX`` to a value greater than 1 to enable 2D decomposition.
    Note that 2D decomposition does not yet work with GNU compilers, so this value will be reset to 1 automatically when using GNU compilers (i.e., when ``COMPILER: gnu``).
 
 RUN_PRDGEN Configuration Parameters