storage: Add document for possible problem on volume detection

For pool which relies on remote resources, such as a "iscsi" type
pool, since how long it takes to export the corresponding devices
to host's sysfs is really depended, it could depend on the network
connection, it also could depend on the host's udev procedures. So
it's likely that the volumes are not able to be detected during pool
starting process, polling the sysfs doesn't work, since we don't
know how much time is best for the polling, and even worse, the
volumes could still be not detected or partly not detected even after
the polling.  So we end up with a documentation to prompt the fact,
in virsh manual.

And as a small improvement, let's explicitly say no LUNs found in
the debug log in that case.

src/storage/storage_backend_scsi.c

@@ -495,6 +495,7 @@ virStorageBackendSCSIFindLUs(virStoragePoolObjPtr pool,
     DIR *devicedir = NULL;
     struct dirent *lun_dirent = NULL;
     char devicepattern[64];
+    bool found = false;
 
     VIR_DEBUG("Discovering LUs on host %u", scanhost);
 
@@ -516,11 +517,15 @@ virStorageBackendSCSIFindLUs(virStoragePoolObjPtr pool,
             continue;
         }
 
+        found = true;
         VIR_DEBUG("Found LU '%s'", lun_dirent->d_name);
 
         processLU(pool, scanhost, bus, target, lun);
     }
 
+    if (!found)
+        VIR_DEBUG("No LU found for pool %s", pool->def->name);
+
     closedir(devicedir);
 
     return retval;

tools/virsh.pod

@@ -2650,6 +2650,15 @@ Refresh the list of volumes contained in I<pool>.
 
 Start the storage I<pool>, which is previously defined but inactive.
 
+B<Note>: A storage pool that relies on remote resources such as an
+"iscsi" or a (v)HBA backed "scsi" pool may need to be refreshed multiple
+times in order to have all the volumes detected (see B<pool-refresh>).
+This is because the corresponding volume devices may not be present in
+the host's filesystem during the initial pool startup or the current
+refresh attempt. The number of refresh retries is dependant upon the
+network connection and the time the host takes to export the
+corresponding devices.
+
 =item B<pool-undefine> I<pool-or-uuid>
 
 Undefine the configuration for an inactive I<pool>.