docs/sysfs: Update directory/kobject documentation.

Some time ago the way how sysfs stores a pointer to a kobject
corresponding to a directory was modified. This patch brings the
documentation again in sync with the implementation.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>