virNodeGetInfo: Improve description of the case when fake data is reported

virNodeGetInfo: Improve description of the case when fake data is reported

virNodeGetInfo due to the rigid desing of the filled struct can't
faithfully represent all topologies. Improve the description when that
happens and outline the fallback topology.

The function docs already state that users ought to use
virConnectGetCapabilities() instead.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>