I think most of the answers are missing the essence of your dilemma.
You didn't have a great work environment and you felt slighted at the company, so naturally you found another job.
Now that you are leaving they seem to value what you did more. You may think your task are simple but your management may be freaking out. So what right?
First they can ask you to do whatever you want during your notice period. You can refuse most or all that don't have to do with your current job.
You may think you want to stick it to the company but you are also sticking it to your co-workers a bit too. They may have trouble functioning, they might have to work extra, maybe they don't but they remember you left in a hostile manner. So even if you don't want anything to do with this company for the rest of your life you now have 20-30 people in your industry that view you as a black cloud. So step one is play nice and have a good attitude your last few weeks since people will remember those the most.
Next have a talk with your manager and be honest about what you can get done. For example you mention teaching your coworkers to train them to your level. I personally see issues with this. I have been in tech for 20 years and had 40+ guys reporting to me at a time. I don't ever remember asking one to train their replacement when they gave notice. You are not a trainer or teacher of developers and if you were you would get paid more.
Asking you to document what you do is fine. If you want to do a half-ass job, whatever. Just have a good attitude about it and don't act like you are half-assing it. Any level headed tech manager worried about an employee leaving with knowledge first has already failed their job and second they would have someone shadow you do specific things - note I said shadow not have you train them. That puts the onus of when things get done on the manager not you. No manager should trust that you will get this stuff done when you are on notice so they should be managing what is being transferred. The same reason you wanted to leave is probably the same reason they are mismanaging this.
Also don't let your management or HR rile you up. If they want to act like you are being hard to deal with just smile and do your job. It is a tough situation leaving a company that has management issues. Don't focus on getting everything done. Let them manage that. Offer to be available after notice period (for a premium). If they don't want to take you up on it, that is on them. My gut tells me they will let their junior devs run around in circles for months.