waterway - edit docs

layers/waterway/waterway.yaml

@@ -1,10 +1,13 @@
 layer:
   id: "waterway"
   description: |
-      OpenStreetMap [waterways](https://wiki.openstreetmap.org/wiki/Waterways) for higher zoom levels
-      and Natural Earth rivers and lake centerlines for low zoom levels.
+      OpenStreetMap [waterways](https://wiki.openstreetmap.org/wiki/Waterways) for higher zoom levels (z9 and more)
+      and Natural Earth rivers and lake centerlines for low zoom levels (z3 - z8).
       Linestrings without a name or which are too short are filtered
-      out at low zoom levels. Waterways do no not have a `subclass` field.
+      out at low zoom levels.
+      Till z11 there is `river` class only, in z12 there is also `canal` generated,
+      starting z13 there is no generalization according to `class` field applied.
+      Waterways do not have a `subclass` field.
   buffer_size: 4
   fields:
     name: |