first stab at describing the new raster environment variables

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12506 b70326c6-7e19-0410-871a-916f4a2858ee
slight revisions to example

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12507 b70326c6-7e19-0410-871a-916f4a2858ee