Leona/doc static (#3519)

* Static linking refactorization

* File name changed

* Doc for PR 3497 - improve static backend apis

* Merge issue

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

 .. backends/backend-api/index.rst:
 
 
-Backend
-=======
+.. _backend_api_macros:
+
+Backend APIs
+############
+
+Each backend ``BACKEND`` needs to define the macro ``${BACKEND}_API`` 
+appropriately to import symbols referenced from outside the library, 
+and to export them from within the library. See any of the
+``${backend}_backend_visibility`` header files for an example; see 
+also :ref:`what_is_backend`
+
+.. 
+
 
 .. doxygenclass:: ngraph::runtime::Backend
    :project: ngraph

doc/sphinx/source/backends/index.rst

@@ -34,6 +34,13 @@ from source in our documentation: :ref:`ngraph_plaidml_backend`.
    AMD\* GPUs, Yes, Some
 
 
+Each backend must define a function ``ngraph_register_${backend}_backend`` 
+that registers a backend constructor function and ensures that 
+initializations are performed. An example that includes initializations 
+can be found in the ``ngraph/src/runtime/cpu/cpu_backend.cpp`` file. See 
+also: :ref:`backend_api_macros`.
+
+
 .. _how_to_use:
 
 How to use?