Add docs for landcover, landuse and housenumber

layers/landcover/landcover.yaml

@@ -1,7 +1,17 @@
 layer:
   id: "landcover"
-  description: Landcover is used to describe the physical material at the surface of the earth. Land covers include grass, asphalt, trees, bare ground etc. Tagging of landcover is often only implied by other tags at present, for example a park may be assumed to be covered in grass, but in some places this may in fact be trees or sand or some other cover. The following keys imply or specifically indicate landcover properties.
+  description: |
+      Landcover is used to describe the physical material at the surface of the earth. At lower zoom levels this is
+      from Natural Earth data for glaciers and ice shelves and at higher zoom levels the landcover is implied
+      by OSM tags (like `landuse` or `natural`).
   buffer_size: 4
+  fields:
+    class: |
+        More generalized groups of either `farmland`, `ice`, `wood`, `grass` or `wetland`.
+        Use this to assign general colors to the landcover.
+    subclass: |
+        Original value of either the `landuse`, `natural` or `wetland` tag.
+        Use this to do more precise styling.
   datasource:
     geometry_field: geometry
     query: (SELECT geometry, class, subclass FROM layer_landcover(!bbox!, z(!scale_denominator!))) AS t