The manual is right: it takes a while for disk expansion to complete. I added two more 500GB disks to my gear. The first message I got when I powered up the device was:

One or more expansion disks will be initialized and scanned for bad blocks.

About 2.5 hours later, the NV said:

Disk initialization successfully finished.  At this time you can reboot the device to complete the volume expansion process.

whereupon I rebooted the NAS. That was five hours ago, and the expansion is still running, but I’m confident it will finish sometime.