[ietf-dkim] Issue 1553: note on figure in overview draft

[Stephen Farrell]

Issue description: https://rt.psg.com/Ticket/Display.html?id=1553

I'm not sure that we converged totally on this, but the thread [1]
does have proposed changes, so I guess we have to wait until
the next rev of the overview, at which point we can hopefully
close this. OTOH if Jim & John are ok that the changes suggested
in the thread is ok, then I'd be ok with closing it now.

S.

[1] http://mipassoc.org/pipermail/ietf-dkim/2008q1/009634.html