doc: Document quirk of getting block job info for a 'backup' blockjob

The stats reported for a blockjob which is member of a domain pull
backup refer to the utilization of the scratch file rather than the
progress of the backup as the progress of the backup depends on the
client. Note this quirk in the docs.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/libvirt-domain.c

@@ -9949,6 +9949,10 @@ virDomainBlockJobAbort(virDomainPtr dom, const char *disk,
  * and was no-op. In this case libvirt reports cur = 1 and end = 1.
  * Since 2.3.0.
  *
+ * Note that the progress reported for blockjobs corresponding to a pull-mode
+ * backup don't report progress of the backup but rather usage of temporary
+ * space required for the backup.
+ *
  * Returns -1 in case of failure, 0 when nothing found, 1 when info was found.
  */
 int

tools/virsh.pod

@@ -994,6 +994,10 @@ I<--bytes> with a scaled value permits a finer granularity to be selected.
 A scaled value used without I<--bytes> will be rounded down to MiB/s. Note
 that the I<--bytes> may be unsupported by the hypervisor.
 
+Note that the progress reported for blockjobs corresponding to a pull-mode
+backup don't report progress of the backup but rather usage of temporary
+space required for the backup.
+
 =item B<blockpull> I<domain> I<path> [I<bandwidth>] [I<--bytes>] [I<base>]
 [I<--wait> [I<--verbose>] [I<--timeout> B<seconds>] [I<--async>]]
 [I<--keep-relative>]