Update-strategies should specify how to use the reboot window feature of locksmith

[brianredbeard]

The page https://github.com/coreos/docs/blob/master/os/update-strategies.md#auto-updates-with-a-maintenance-window specifically tries to configure a shell script to achieve the functionality of https://github.com/coreos/locksmith/#reboot-windows . We should unify this documentation,

[joshix]

related #635 (where we almost did the shell script thing again, averted at the last minute thanks to crawford)