[waf] Access Control Issue 19: Requirements document updated dated 20080114

(sorry, not quite sure the exact format for subject lines) 
Latest version will always be available at 
http://dev.w3.org/2006/waf/access-control/AccessControl-Requirements
 
and most recent dated version is:
http://dev.w3.org/2006/waf/access-control/AccessControl-Requirements-200
80114.html
 
I added the 3 usage scenarios from Art's presentation and 1 from Jon
Ferraiolo.  I did not edit the requirements as I couldn't detect a
consensus opinion yet.
 
Cheers,
Dave