add hint for installing debian in an LXC container

[nerdoc]

chronyd doesn't start there correctly without the -x option.
fixes openmediavault/openmediavault#1678

[ryecoaaron]

This basically defeats the purpose of chrony. It can't set the system clock when -x is passed. This would be very bad for everything except for LXC which is not supported.

-x
This option disables the control of the system clock. chronyd will not try to make any adjustments of the clock. It will assume the clock is free
running and still track its offset and frequency relative to the estimated true time. This option allows chronyd to be started without the
capability to adjust or set the system clock (e.g. in some containers) to operate as an NTP server.

[votdev]

I don't think it makes sense to add a note for a container setup that is not supported by OMV.