Added a not to flag limit "errorIfIsOverLimit" method

refs https://github.com/TryGhost/Team/issues/510

- Flag limits are impossible to check if they are "over a limit already" as they are just that - on/off flags. Therefore it should be directly noted that the method is there to keep the "Limit" interface and not be relied upon

ghost/limit-service/lib/limit.js

@@ -150,7 +150,8 @@ class FlagLimit extends Limit {
     }
 
     /**
-     * Flag limits are on/off so we can't be over the limit
+     * Flag limits are on/off. They don't necessarily mean the limit wasn't possible to reach
+     * NOTE: this method should not be relied on as it's impossible to check the limit was surpassed!
      */
     async errorIfIsOverLimit() {
         return;