Move ST_Retile documentation under "raster constructors"

Add note about difference from ST_Tile, link ST_Tile and ST_Resample

git-svn-id: http://svn.osgeo.org/postgis/trunk@12972 b70326c6-7e19-0410-871a-916f4a2858ee
Remove ST_Tile references from ST_Retile docs

... wasn't very useful that way ...

git-svn-id: http://svn.osgeo.org/postgis/trunk@12973 b70326c6-7e19-0410-871a-916f4a2858ee