Add possible values for landcover, landuse, park doc

layers/park/park.yaml

@@ -4,8 +4,12 @@ layer:
       The park layer contains parks from OpenStreetMap tagged with either [`boundary=national_park`](http://wiki.openstreetmap.org/wiki/Tag:boundary%3Dnational_park) or [`leisure=nature_reserve`](http://wiki.openstreetmap.org/wiki/Tag:leisure%3Dnature_reserve).
   buffer_size: 4
   fields:
-    class: |
-        Either `national_park` or `nature_reserve`.
+    class:
+      description: |
+          Use the **class** to differentiate between different parks.
+      values:
+      - national_park
+      - nature_reserve
   datasource:
     geometry_field: geometry
     query: (SELECT geometry, class FROM layer_park(!bbox!, z(!scale_denominator!))) AS t