This is not a feature specific to the mailserver. Indeed, the feature
was added to `system.autoUpgrade.allowReboot` with NixOS 19.09 and it
has better detection if a reboot is necessary.
For the system.autoUpgrade there is no kexec option, but the use was
discouraged.
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.