Documentation: coding-style: Document global variable guidelines

Document the issue related to global variable dependencies and how to
avoid them.

Signed-off-by: Paul Elder <paul.elder@ide# SPDX-License-Identifier: GPL-2.0-or-later
#
# Copyright (C) 2022, Paul Elder <paul.elder@ideasonboard.com>
#
# Gradients that can be used to distribute or map numbers

import libtuning as lt

import math
from numbers import Number


# @brief Gradient for how to allocate pixels to sectors
# @description There are no parameters for the gradients as the domain i