devices.txt: improve entry for comedi (char major 98)

Describe how the comedi minor device numbers are split across comedi
devices and comedi subdevices.

Replace the current, long dead URL with an official URL for the Comedi
project.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>