lib: Document limitation of virDomainInterfaceAddresses

https://bugzilla.redhat.com/show_bug.cgi?id=1588336

This API takes @source argument which tells it where to get
domain IP addresses from. However, not all sources are capable of
providing all the information we report, for instance ARP table
has no notion of IP address prefixes. Document this limitation.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NLaine Stump <laine@laine.org>

src/libvirt-domain.c

@@ -11780,6 +11780,10 @@ virDomainFSInfoFree(virDomainFSInfoPtr info)
  * be unreachable. Depending on the route table config of the
  * guest, the returned mac address may be duplicated.
  *
+ * Note that for some @source values some pieces of returned @ifaces
+ * might be unset (e.g. VIR_DOMAIN_INTERFACE_ADDRESSES_SRC_ARP does not
+ * set IP address prefix as ARP table does not have any notion of that).
+ *
  * @ifaces->name and @ifaces->hwaddr are never NULL.
  *
  * The caller *must* free @ifaces when no longer needed. Usual use case