Update top-level-api.md (#6056)

Show transparent overlay option in docs

docs/docs/top-level-api.md

     options: {
       overlay: {
         interceptTouchOutside: true
-      }
+      },
+      layout: {
+        componentBackgroundColor: 'transparent', // Use this if you want your background to be transparent.
+      },
     }
   }
 });