Yes, I definitely agree it's useful as a doc comment.

My worry about showing internal variable/configuration names to people is that it creates the appearance that it's an actual supported API. And if people (or simply popular plugins) rely on the way we store our configuration, we'd never be able to change it. So I'd rather not encourage people to access these variables - though I have nothing against people monkey-patching their own plugins (which I've done many times), it's done entirely at their own risk. :)

You also bring up a good point about this only executing once, so there's the question: for the small subset of people who this message would be useful to, will they ever even see it? I suppose that makes both of our arguments moot, because if no one sees it then it can't be harmful, but it also can't be useful. :)