The section on Additional meta-data in Doc/distutils/setupscript.rst is
missing any reference to the setup() "platforms" keyword. I've attached
a patch which fills in the basics but there are some outstanding questions:

 - Does note (4) about backwards compatibility with earlier versions of
Python apply?

 - What strings should be used to describe each platform? Perhaps any
final element of an "Operating System ::" trove classifier?

 - Given that we have the classifiers, is the "platforms" keyword
actually needed? Perhaps the docs should just mark it as deprecated? We
would then maybe need to change distutils to populate the platforms key
in PKG-INFO from the classifiers?