CD-1 issue #45: obligation vs obligation expression

The issue number refers to the XLS-sheet found in this email:
http://lists.oasis-open.org/archives/xacml/200909/msg00013.html

The commenter notes that the terms obligation and obligation expression 
are used inconsistently across the specification.

I agree. This should be clarified now that obligation and obligation 
expression are separate.

I propose that we fix this. I can make the changes as word diffs so you 
can review them, rather than posting a long list of individual changes here.

Best regards,
Erik