Make zoom level doc of transportation less strict

layers/transportation/transportation.yaml

@@ -3,12 +3,11 @@ layer:
   description: |
       **transportation** contains roads and railways.
       This layer is directly derived from the OSM road hierarchy.
-      Only at zoom level 4 to 6 some major highways from Natural Earth
-      are used.
+      At lower zoom levels major highways from Natural Earth are used.
       It contains all roads from motorways to primary, secondary and
       tertiary roads to residential roads and
       foot paths. Styling the roads is the most essential part of the map.
-      The `transportation` layer also contains polygons for things like plazas.
+      The `transportation` layer also contains polygons for features like plazas.
   buffer_size: 4
   srs: +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over
   fields: