more default-meaning and minor doc fixes
more default-meaning and minor doc fixes

CartoDB/mapnik-reference,mapnik/mapnik-reference,mapnik/mapnik-reference,mapycz/mapnik-reference,florianf/mapnik-reference,mapycz/mapnik-reference,mojodna/mapnik-reference,CartoDB/mapnik-reference,mojodna/mapnik-reference,florianf/mapnik-reference,mapnik/mapnik-reference