Cherrypick more doc from PR 3104 to avoid jam

doc/sphinx/source/backends/backend-api/index.rst

@@ -4,7 +4,68 @@
 Backend
 =======
 
+:ref:`more_resources`
+
 .. doxygenclass:: ngraph::runtime::Backend
    :project: ngraph
    :members:
 
+
+
+.. _more_resources: 
+
+Additional resources
+====================
+
+OpenCL
+------
+
+#. Install the latest Linux driver for your system from 
+   https://software.intel.com/en-us/articles/opencl-drivers;
+   You may need to install `OpenCL SDK`_ in case of an 
+   libOpenCL.so absence.
+
+#. Any user added to "video" group: 
+
+   .. code-block:: console 
+
+      sudo usermod –a –G video <user_id>
+
+   may, for example, be able to find details at the
+   ``/sys/module/[system]/parameters/`` location. 
+
+
+
+nGraph Bridge 
+~~~~~~~~~~~~~
+
+When specified as the generic backend -- either 
+manually or automatically from a framework --  
+``NGRAPH`` defaults to CPU, and it also allows 
+for additional device configuration or selection. 
+
+Because nGraph can select backends, you may try 
+specifying the ``INTELGPU`` backend as a runtime 
+environment variable: 
+
+:envvar:`NGRAPH_TF_BACKEND="INTELGPU"`
+
+An `axpy.py example`_ is optionally available to test;
+outputs will vary depending on the parameters
+specified. 
+
+.. code-block:: console
+
+   NGRAPH_TF_BACKEND="INTELGPU" python3 axpy.py
+
+* ``NGRAPH_INTELGPU_DUMP_FUNCTION`` -- dumps 
+  nGraph’s functions in dot format.
+
+* `` `` --.
+
+* `` `` --.
+
+* `` `` --.
+
+.. _axpy.py example: https://github.com/tensorflow/ngraph-bridge/blob/master/examples/axpy.py
+.. _OpenCL SDK: https://software.intel.com/en-us/opencl-sdk
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