Add notes to findByHeadcode(headcode)

Signed-off-by: Fred Boniface <fred@fjla.uk>

src/services/trainService.services.js

@@ -14,6 +14,8 @@ async function findByHeadcode(headcode) {
     scheduleEndDate: {$gte: now},
     daysRun: {$in: [shortDay]}
   };
+  // At this point, the returned objects need sorting to ensure the correct object is returned.
+  // C, N, O, P is the order - C being the preferred, then N, then O and finally P.
   return db.query('timetable', query);
 }