FIX-#3732: Change doc for native+omnisci (#3733)

Signed-off-by: Maria Rubtsova <maria.rubtsova@intel.com>

docs/UsingOmnisci/index.rst

@@ -5,24 +5,7 @@ This section describes usage related documents for the OmniSciDB-based engine of
 
 This engine uses analytical database OmniSciDB_ to obtain high single-node scalability for
 specific set of dataframe operations.
-To enable this engine you could set the following environment variables:
-
-.. code-block:: bash
-
-   export MODIN_ENGINE=native
-   export MODIN_STORAGE_FORMAT=omnisci
-   export MODIN_EXPERIMENTAL=true
-
-or turn it on in source code:
-
-.. code-block:: python
-
-   import modin.config as cfg
-   cfg.Engine.put('native')
-   cfg.StorageFormat.put('omnisci')
-   cfg.IsExperimental.put(True)
-
-To enable ``OmniSci`` engine launch export :
+To enable this engine you can set the following environment variable:
 
 .. code-block:: bash
 
@@ -35,4 +18,7 @@ or use it in your code:
    import modin.config as cfg
    cfg.StorageFormat.put('omnisci')
 
+Since OmniSci is run through its native engine Modin itself sets ``MODIN_ENGINE=Native``
+and you might not specify it explicitly.
+
 .. _OmnisciDB: https://www.omnisci.com/platform/omniscidb