Use "vdev" instead of "devices" when referring to vdevs

Update documentation to use the correct terminology.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: trick2011 <trick2011@users.noreply.github.com>
Closes #17734
Closes #17755