Add docs for landcover, landuse and housenumber

layers/housenumber/housenumber.yaml

@@ -1,11 +1,12 @@
 layer:
   id: "housenumber"
   description: |
-      [OpenStreetMap housenumbers]
+      Everything in OpenStreetMap which contains a `addr:housenumber` tag useful for labelling housenumbers on a map.
+      This adds significant size to *z14*. For buildings the centroid of the building is used as housenumber.
   buffer_size: 8
   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:
-    class: String
+    housenumber: Value of the [`addr:housenumber`](http://wiki.openstreetmap.org/wiki/Key:addr) tag.
   datasource:
     geometry_field: geometry
     srid: 900913