fix pylint warning '(pointless-string-statement) String statement has no effect'

Python doesn't support docstring for variables directly. Where
appropriate Sphinx style for docstrings for variables is used
'#: some-docstring'

.. Beremiz documentation master file

Beremiz's documentation
=======================

Contents:

.. toctree::
   :maxdepth: 2

   overview
   manual/index
   standards