Backport sfcgal cleanup fixes from #2779, #2753 (note for this just removed ST_IsPlanar from doco since it was never released until 2.2 and corrected spelling of ST_MinkowskiSum in doco), fixed comments generation script

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12646 b70326c6-7e19-0410-871a-916f4a2858ee