getstats: improve documentation

getstats: improve documentation

At least with 'virsh domstats --block' on an offline domain, we
currently output no stats even though we recognize the stat
category.  Although a later patch will improve this situation,
it is better to document that this is expected behavior.

Also, while the current implementation rejects filtering flags
for virDomainListGetStats, this limitation may be lifted in the
future and we do not enforce it at the API level.

* src/libvirt-domain.c (virConnectGetAllDomainStats): Document
that recognized stats might not be reported.
(virDomainListGetStats): Likewise, and tweak filtering documentation.

Signed-off-by: Eric Blake <eblake@redhat.com>