Info about SLO on IPDv4 - no final logout response

[Cantor, Scott]

> I expected the idp to send a logout response to the SP_A to conclude, but this
> doesn't happen.

Shibboleth SPs may be issuing logout requests with the Asynchronous extension element that tells an IdP it doesn't need to respond.

SPs are obligated to take care of all clean up before issuing a logout request to an IdP, so it is superfluous to even bother with a response and the extension allows that to be signalled.

-- Scott