fixing doc build issues, errors, and providing docstring decorator

[dlyssenko]

fixed some doc build issues and errors
provided docstring decorator:

• Docstrings cannot be interpolated. However, it's better to refer in the docstrings to defined constants by names rather than by values, so this decorator provides that functionality - it interpolates enumerated symbols. That way the docstrings in the source will refer to the constants by names, while upon building it will resolve and interpolate the values.