Add availability info for {Add,Drop}OverviewConstraints

git-svn-id: http://svn.osgeo.org/postgis/trunk@12933 b70326c6-7e19-0410-871a-916f4a2858ee
Add documentation for {Add,Drop}OverviewConstraints (#2824)

git-svn-id: http://svn.osgeo.org/postgis/trunk@12934 b70326c6-7e19-0410-871a-916f4a2858ee