The leaverbuster system is meant to be forgiving and not give punishments that are meaningful, just small wait times, so that it can "punish" the people who do leave without punishing for real those who just don't understand or have connection issues. It's frustrating at first and you're probably right that there needs to be a tooltip, but rest assured that you won't get any meaningful punishments unless it becomes a purposeful habit.
You can get leaverbuster penalties from:
-Not locking in your champion
-"Dodging" a game by exiting the client
- Leaving a game while it is still in progress
Boards can be a little harsh to newcomers and won't always answer any questions, but if you need anything else explained or help with something, you can hmu and message my Weathered account.