#2920 stab at explaining requirement for alignment and add more detail to ST_SameAlignment

git-svn-id: http://svn.osgeo.org/postgis/trunk@12952 b70326c6-7e19-0410-871a-916f4a2858ee
wrong function (non-existent xref)

git-svn-id: http://svn.osgeo.org/postgis/trunk@12953 b70326c6-7e19-0410-871a-916f4a2858ee