[xacml-comment] ...IsPresent and Qualified...

In draft 18f section 5.30, 5.31, and 5.32 documents the 
AttributeIsPresent elements,
but the 18f schema doesn't contain these.

Also, the 18f schema contains the QualifiedSubjectAttributeDesignator 
element, but
this isn't described in the 18f draft, it first appears in the 
conformance tables 10.3.1

BTW: I think the 18f draft is in really good shape.

John