Make synopsis scrollable on wide screens

When the synopsis is longer than the screen, you can’t see its end
and you can't scroll down either, making the content unreachable.

haddock-api/resources/html/NewOcean.std-theme/new-ocean.css

@@ -138,6 +138,7 @@ body.js-enabled .hide-when-js-enabled {
   #synopsis .show {
     border: 1px solid #5E5184;
     padding: 0.7em;
+    max-height: 65vh;
   }
 }