Actually, it WON"T update automatically...
It will prompt you that an update is available, then you must select the "I'm In" button and the upgrade will download in the background when on WiFi... This update is 1.2GB and there is no option to download on mobile data, it must be on WiFi. After the download is completed, there will be a message that the update is ready to apply and the usually warning that it will be unavailable, no 911, etc, for quite a while, after you accept the terms then the phone will reboot and apply the updates.
If you do not select the "I'm In" button, it will wait (I believe 3 or 5 days the first time) before it prompts you again, I think even a manual check will "fail" during this windows but I don't remember for sure, then every 24 hours after that it will remind you of an update.
Moto devices do not force you to take an update, or do it unattended. But it will annoy you into taking it.
The other question asked once in a while is "What will happen if I don't have 1.2GB of space available?" and actually it needs a little more than that, I don't think anyone has said for certain but there is this comment on Lenovo's forums. But it seems there is some space reserved that is released after the final major OS upgrade.