formatdomain.html.in: document the NUMA cpus auto fill feature

formatdomain.html.in: document the NUMA cpus auto fill feature

We're not mentioning that we're replicating QEMU behavior on purpose.
First because QEMU will one day, maybe, change the behavior and
start to refuse incomplete NUMA setups, and then our documentation
is now deprecated. Second, auto filling the CPUs in the first
cell will work regardless of QEMU changes in the future.

The idea is to encourage the user to provide a complete NUMA CPU topology,
not relying on the CPU auto fill mechanic.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>