docs/driver-model: Document device.groups

Several drivers use device_create_file() where device.groups should be
used instead. This patch documents that and also removes the comments
about device classes since these should not be used in new code in the
way documented until now in Documentation/driver-model/device.txt.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>