This patch is part of addressing setuptools issue 2492:
pypa/setuptools#2492

The code in this Note block was found to not apply to equivalently-named
features in setup.py.  After discussion on the above issue, a project
maintainer noted this behavior was expected.  This patch adds
documentation of expected behavior close to where it would be discovered
by somebody who stumbled upon the attr: feature, but might not know the
background on directives and setup.cfg vs. setup.py.

Thanks to @webknjaz for the style pointer.

Co-authored-by: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua>