qemu-options: Rewrite -numa documentation

Rewrite the -numa documentation to clarify what exactly it does.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170123180632.28942-3-ehabkost@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>