Let's use --absolute because it is less error-prone: I originally
thought the number there was the *final* disk space. But actually,
without --absolute, it is *added* to the existing disk space. This can
lead to fairly catastrophic results if we add too much disk space, as
reverting this requires a complex backup / restore procedure of the
VM, because grow-disk doesn't support shrinking disks.

Also mention this problem in the docs.