[PATCH] docs: Add nanoseconds units for vcpu.delay doc

Submitted https://gitlab.com/libvirt/libvirt/-/merge_requests/352: Add nanoseconds units for vcpu.delay doc, as it's based on /proc/<pid>/task/<tid>/schedstat, according to https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_driver.c#L128.... schedstat is in nanoseconds units, according to https://docs.kernel.org/scheduler/sched-stats.html#proc-pid-schedstat.