Since NixOS 19.09 autoUpgrade also has the ability to do automatic
reboots. Its detection on whether a reboot is necessary is a bit more
sophisticated. Having this option in the mail-server implied to me that
it did something additionally, though it was just a feature which was
not included in NixOS at the time it was introduced for the mail-server.
Mentioning the fact in the documentation might help people not to get
confused why they should turn the `system.autoUpgrade.allowReboot` off
and instead use the mail-servers reboot flag.
