doc: setting [compute]max_disk_devices_to_attach = 0 prevents instances from booting

This is a bug to track the addition of a note to the config option help for [compute]max_disk_devices_to_attach. An issue was raised downstream [1] around the fact that setting this option to 0 will cause instances not to boot because they will be allowed 0 disk devices.

The CONF.max_disk_devices_to_attach option controls the maximum number
of disk devices allowed to attach to an instance. If it is set to 0, it
will literally allow no disk device for instances, preventing them from
being able to boot.

There doesn't appear to be any way to disallow particular values
in a oslo.config IntOpt other than the min/max values. Here we need the
min value to be -1 to represent unlimited. There is a 'choices' kwarg
available but that is only for enumerating valid values and we need to
allow any integer >= 1 as well. Finally, there doesn't seem to be a
good way to validate this option setting as -1 or >= 1 because it's a
nova-compute config option and our choices there are likely: prevent
nova-compute from starting because of value 0 or log a WARNING about
it, both of which seem like they might be overkill. So, highlighting
the behavior of value 0 in the documentation might be the best way to
handle the issue.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1801931