doc update with regards to #1908

In order to make C-module dependencies work on RTD they need not only to be mocked by also removed from setup.py to prevent installation attempts.

Author: chiffa

docs/faq.rst

@@ -66,6 +66,8 @@ Of course, replacing `MOCK_MODULES` with the modules that you want to mock out.
 
         from mock import Mock as MagicMock
 
+If such libraries are installed are installed via ``setup.py``, you also will need to remove all the C-dependent libraries from your ``install_requires`` in the RTD environment.
+
 `Client Error 401` when building documentation
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