1554914 make olink targetdoc attribute optional

Hi Norm,
We approved this at the last meeting, but the change doesn't seem to have 
gotten into DB 5.0 beta 8.  Was that an oversight?

Also, beta 8 RNC shows it as data type xsd:anyURI, and I think it should 
just be text.

Bob Stayton
Sagehill Enterprises
DocBook Consulting