XLink Note updated

I've just checked in rev 1.5. Changes:

 - Phrase "SML XLink Reference Scheme" is used consistently throughout
 - Restored some square brackets around abstract property names that
   had been accidentally dropped
 - Added a ref to XML Infoset after mention of [base URI] property;
   this is consistent with the SML spec
 - Completed the References section

See http://dev.w3.org/cvsweb/2007/xml/sml/xlink-note.html.diff?r1=1.4&r2=1.5&f=h for visual diff.

-L