Is there any official documentation on how the auto discharge feature works for the Mavic 2 Zoom?  

Here are my questions:  

• When and how does the battery get the setting for discharge time?  When it's in the aircraft and the discharge time is set?  Or just when it's in the aircraft and it gets powered up?
• If we have multiple batteries, and we change the time in the app, how do we update the discharge time in the batteries?
• Does the battery need to be in the aircraft in order to discharge?
  

The setting for discharge time is stored in the aircraft. When a battery is inserted in the aircraft, the aircraft updates the discharge time and the value is sent to the battery. The default setting is 5 days, and that is perfecly adequate. The battery will discharge automatically when it is out of the aircraft, as in sitting in a carrying case. The countdown time starts after the battery was last used or was last in the aircraft. If you press the battery status button, it resets the countdown to zero again, so best to not press the button. If you have multiple batteries, place them in the aircraft to update the time.

When and how does the battery get the setting for discharge time?  When it's in the aircraft and the discharge time is set?  Or just when it's in the aircraft and it gets powered up?
You do it by going into the app and going to the battery settings.
You have to set the discharge time for each battery individually.

Does the battery need to be in the aircraft in order to discharge?
No

Hi, just to answer your inquiries:
1. To set the auto discharge time, you need to put the battery in the drone and set it in battery settings. You may check each battery's auto discharge time by changing the battery and check it in the battery settings.
2. If you need to change the auto discharge time for batteries, please swape the batteries and set them in the app.
3. Batteries don't need to be put in the drone for auto discharge.
Actually captains above provided great answers already. Hope this provides you with the info you needed.