Docs build error (google vs numpy docstrings)

[tkknight]

📚 Documentation

The "make linkcheck" on master as of 11/09/20 shows an odd error on travis, see https://travis-ci.org/github/SciTools/iris/jobs/725551522#L1237.

Build issue shows as:

Reading sources... [100%] whatsnew/latest
/home/travis/miniconda/envs/iris-dev/lib/python3.7/site-packages/scitools_iris-3.0.dev0-py3.7.egg/iris/palette.py:docstring of iris.palette.SymmetricNormalize.process_value:6: WARNING: Unexpected section title.
Returns
-------
/home/travis/miniconda/envs/iris-dev/lib/python3.7/site-packages/scitools_iris-3.0.dev0-py3.7.egg/iris/palette.py:docstring of iris.palette.SymmetricNormalize.process_value:13: WARNING: Unexpected section title.
Notes
-----
looking for now-outdated files... none found

I believe this is due to the Iris code base using Google Style docstrings but the code that is pulled in above is from matplotlib that now uses NumPy docstrings, see https://matplotlib.org/_modules/matplotlib/colors.html#Normalize.process_value.

#3840 may resolve this.