Update style_guide.rst

Author: echedey-ls

docs/sphinx/source/contributing/style_guide.rst

@@ -70,6 +70,25 @@ the ``docs/readthedocs.org:pvlib-python`` link within the checks
 status box at the bottom of the pull request.
 
 
+.. _documentation-units:
+
+Parameter names and units
+-------------------------
+
+When specifying parameters and their units, please follow these guidelines:
+- Use the recommended parameter name and units by :ref:`variables_style_rules` where possible.
+- Enclose units in square brackets, e.g. ``[W]``.
+- Use unicode superscripts symbols for exponents, e.g. ``m²``.
+
+  These superscripts characters are ``⁰``, ``¹``, ``²``, ``³``, ``⁴``, ``⁵``, ``⁶``, ``⁷``, ``⁸``, ``⁹``, ``⁺``, ``⁻``. Degree symbol is ``°``.
+
+- Link to a brief description of the magnitude in the :ref:`variables_style_rules` section if it exists, via the sphinx role |:term:`glossary_term`|. For example, to document ``dni`` use:
+
+  .. code-block:: rst
+
+      dni : numeric
+          Direct normal irradiance, see :term:`dni`. [Wm⁻²]
+
 .. _building-the-documentation:
 
 Building the documentation