docs(shipwrecks): describe the layer on its own, not by a layer that is not on main

Air accidents (#1584) is not merged, so comments, the changelog fragment and
CURRENT-STATE no longer explain the shipwrecks markers, card and picker by
it; the row-range test fixture is the shipwrecks row. CURRENT-STATE now says
the markers are one billboard collection and the period is stored swapped.

changelog.d/1583-shipwrecks.md

@@ -1,4 +1,4 @@
 - New Shipwrecks layer (Events): about 8,000 wrecks from EMODnet (CC BY 4.0)
   and Wikidata (CC0). Each has a card with deaths, date, vessel, cause, depth,
   position quality and a source link. Markers grow with the death toll, and
-  the same period picker as air accidents filters by year (IMBrandao, #1583).
+  a start and end year picker filters by period (IMBrandao, #1583).