Potential Errata: Session Index on logout

The language surrounding session index on the <LogoutRequest>
(line 2546) is a bit lacking.  I suggest we replace that line with
something along the lines of:

  
The index of
the session between the pricipal identified by the subject and the
authenticating authority.   This must correlate to the
<SessionIndex> that was present in the <AuthnStatement> of
the assertion used in the session that is being terminated by this
<LogoutRequest>, if any.   

    

I also think we
should require that this be specified by the SP when sending an
SP-Initiated LogoutRequest if the assertion had a SessionIndex in it. 
I was unable to find such a statement in the specs.

Conor