#3127 revert back to use sphere distance (spheroid doesn't work with knn recheck...

#3127 revert back to use sphere distance (spheroid doesn't work with knn recheck), document that <-> is on sphere, change tests to be based on sphere instead of spheroid., #3131 reverting fixes index returned tuples wrong order issue

git-svn-id: http://svn.osgeo.org/postgis/trunk@13593 b70326c6-7e19-0410-871a-916f4a2858ee