[ietf-dkim] Issue 1558: overview document does not mention message validity

[Stephen Farrell]

Issue description: https://rt.psg.com/Ticket/Display.html?id=1558

Thread: http://mipassoc.org/pipermail/ietf-dkim/2008q1/009735.html

Modulo some wordsmithing this seemed to attract support on the
list, so the editors should make an addition along the lines
suggested.

Barring objection, to be closed one week after overview-10 issues.

S.