Code review comment for lp:~smoser/curtin/trunk.lp1680591

This is good. I think we should also document why the udevadm trigger is required. Grub relies upon udevadm information to confirm UUID presense prior to rebooting into a system which may not see the UUID. Separately, udev does not currently *inotify watch* bcache devices like it does other block devs and therefor we do not get an automatic udevadm trigger when mkfs closes a bcache device (like we do with say a partition).