Improve docs of water and water_name

layers/water_name/water_name.yaml

@@ -7,8 +7,11 @@ layer:
   fields:
     name: The OSM [`name`](http://wiki.openstreetmap.org/wiki/Key:name) value of the water body.
     name_en: The english `name:en` value if available.
-    class: |
-        At the moment only `lake` since no ocean parts are labelled. *Reserved for future use*.
+    class:
+      description: |
+          At the moment only `lake` since no ocean parts are labelled. *Reserved for future use*.
+      values:
+      - lake
   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
   datasource: