vote on issue #151 Add a schematron rule for the callout type constraint

[Resending this with the correct subject line]
Issue
151, Add a schematron rule for the callout type
constraint.
Norm says  In a calloutlist, the only valid
targets for an IDREF are areaset, area, and co. The schema should
include a Schematron rule to enforce this constraint.  https://github.com/docbook/docbook/issues/151
I think this is a useful addition, and I don't see any negative
consequences.  If there are no objections, I move to accept this
change.  Scott can second it and vote, and other voting TC members
can vote.  I vote yes.

--
Bob Stayton