idr: describe how nextidp works in idr_get_next().

It was unclear in original kernel-doc how nextidp worked in
idr_get_next(). Let's describe it.

Signed-off-by: Naohiro Aota <naota@elisp.net>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>