Improve waterway docs

layers/waterway/waterway.yaml

@@ -1,22 +1,24 @@
 layer:
   id: "waterway"
   description: |
-      Rivers based of OpenStreetMap [waterways](http://wiki.openstreetmap.org/wiki/Waterways) for *z9* to *z14*
-      and Natural Earth rivers and lake centerlines from *z3* to *z8*.
+      OpenStreetMap [waterways](http://wiki.openstreetmap.org/wiki/Waterways) for higher zoom levels
+      and Natural Earth rivers and lake centerlines for low zoom levels.
       Linestrings without a name or which are too short are filtered
       out at low zoom levels. Waterways do no not have a `subclass` field.
   buffer_size: 4
   fields:
     name: |
         The OSM [`name`](http://wiki.openstreetmap.org/wiki/Key:name) value of the waterway.
-        The `name` field is empty for NaturalEarth data until **z9** when OSM data is used.
-    class: |
+        The `name` field may be empty for NaturalEarth data or at lower zoom levels.
+    class:
+      description: |
         The original value of the [`waterway`](http://wiki.openstreetmap.org/wiki/Key:waterway) tag.
-        Can be one of [`stream`](http://wiki.openstreetmap.org/wiki/Tag:waterway%3Dstream),
-        [`river`](http://wiki.openstreetmap.org/wiki/Tag:waterway%3Driver),
-        [`canal`](http://wiki.openstreetmap.org/wiki/Tag:waterway%3Dcanal),
-        [`drain`](http://wiki.openstreetmap.org/wiki/Tag:waterway%3Ddrain),
-        [`ditch`](http://wiki.openstreetmap.org/wiki/Tag:waterway%3Dditch)`.
+      values:
+      - stream
+      - river
+      - canal
+      - drain
+      - ditch
   datasource:
     geometry_field: geometry
     query: (SELECT geometry, name, class FROM layer_waterway(!bbox!, z(!scale_denominator!))) AS t