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>

Documentation/admin-guide/devices.txt

@@ -1647,8 +1647,17 @@
 		  0 = /dev/comedi0	First comedi device
 		  1 = /dev/comedi1	Second comedi device
 		    ...
+		 47 = /dev/comedi47	48th comedi device
 
-		See http://stm.lbl.gov/comedi.
+		Minors 48 to 255 are reserved for comedi subdevices with
+		pathnames of the form "/dev/comediX_subdY", where "X" is the
+		minor number of the associated comedi device and "Y" is the
+		subdevice number.  These subdevice minors are assigned
+		dynamically, so there is no fixed mapping from subdevice
+		pathnames to minor numbers.
+
+		See http://www.comedi.org/ for information about the Comedi
+		project.
 
   98 block	User-mode virtual block device
 		  0 = /dev/ubda		First user-mode block device