KDEFrameworkCompilerSettings: add warning note to only use for KF modules
We still need some offer for those wanting newer & stricter settings, but so far we seem to agree that reusing KDEFrameworkCompilerSettings is not the way, so be more official about it in the docs.
PS: Just picked up a formerly started patch and seeing to brush it up until WE for a proposal how to address the before-mentioned need for such offer, this warning is extracted from it :)