Potential Errata: Session Index on logout

["Conor P. Cahill" <concahill@aol.com>]

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