API Docs: Fix generation failing with unused context members

Instead of raising an expection a warnign is generated instead. This 
fixes the issue where `['hair', 'pointcloud']` are disabled for release 
builds. In the future a better solution would be to generate the context 
map dynamically but this would require refactoring of the API: D9988

doc/python_api/sphinx_doc_gen.py

@@ -81,7 +81,7 @@ import sys
 import inspect
 import shutil
 import logging
-import warning
+import warnings
 
 from textwrap import indent