kobject: update the kobject/kset documentation

This provides a much-needed kobject and kset documentation update.

Thanks to Kay Sievers, Alan Stern, Jonathan Corbet, Randy Dunlap, Jan
Engelhardt, and others for reviewing and providing help with this
document.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>