clarify/document what mainDevice is

[diracdeltas]

browser-laptop implements the concept of a main device for sync: https://github.com/brave/browser-laptop/pull/13197/files#diff-23ca389e2bcb77191b5a9c10900eb3a3R559

my understanding is that the main device is the device that, when deleted, sends DELETE_SYNC_USER to the sync server, thereby deleting the entire sync profile on the server.

according to https://github.com/brave/browser-laptop/pull/13197/files#diff-23ca389e2bcb77191b5a9c10900eb3a3R568, the algorithm for determining if a device is a main device is:

1. fetch all devices from the server and save them to local state
2. are any devices already marked as a mainDevice? if so, abort
3. iterate through all the devices and mark any device with the same name as the current device to be a mainDevice.

the sync server has no understanding of main devices; this is only used on the clients. also note that there can be more than one main device according to the algorithm above.

[bryan-snyder]

this is another great idea.