Add warning on support of attr: directive

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.

source/guides/single-sourcing-package-version.rst

@@ -49,6 +49,8 @@ number of your project:
        analysis so that ``attr:`` can function without having to import any of
        the package's dependencies.
 
+       Also, please be aware that declarative config indicators, including the
+       ``attr:`` directive, are not supported in parameters to setup.py.
 
 #.  Use an external build tool that either manages updating both locations, or
     offers an API that both locations can use.