Code review comment for lp:~mbp/launchpad/dkim

I'll add this text, is it more clear:

        """Mail authenticated other than by gpg can create bugs.

        The incoming mail layer is responsible for authenticating the mail,
        and setting the current principal to the sender of the mail, either
        weakly or non-weakly authenticated. At the layer of the handler,
        which this class is testing, we shouldn't care by what mechanism we
        decided to act on behalf of the mail sender, only that we did.

        In bug 643219, Launchpad had a problem where the MaloneHandler code
        was puncturing that abstraction and directly looking at the GPG
        signature; this test checks it's fixed.
        """