Document water and remove obsolete mapping

layers/water/README.md

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+# water
+
+Water polygons representing oceans and lakes. Covered watered areas are excluded (`covered=yes`).
+Until z6 all water originates from Natural Earth. To get a more correct display of the south pole you should also
+style the covering ice shelves over the water.
+Water from OpenStreetMap after z7 is split into many smaller polygons to improve rendering performance.
+This however can lead to less rendering options in clients since these boundaries show up. So you might not be
+able to use border styling for ocean water features.
+
+## Fields
+
+- **class**: Either `ocean`, `lake` or `river`.
+
+## Mapping
+
+![](mapping.png)
+
+