A prior change simply noted "historical reasons" as the source of disparate setup.cfg configuration formats. Add a bit more info to indicate PBR setup.cfg predates setuptools setup.cfg and setuptools simply never adopted some of the functionality that was in PBR. Change-Id: Ifd72dd2a2e26f87dd39274df18e16886b1e196c7 Signed-off-by: Clark Boylan <clark.boylan@gmail.com>