sch_htb: fix doc warning in htb_add_to_id_tree()

Add description for parameters of htb_add_to_id_tree() to fix
gcc W=1 warnings:
net/sched/sch_htb.c:282: warning: Function parameter or member 'root' not described in 'htb_add_to_id_tree'
net/sched/sch_htb.c:282: warning: Function parameter or member 'cl' not described in 'htb_add_to_id_tree'
net/sched/sch_htb.c:282: warning: Function parameter or member 'prio' not described in 'htb_add_to_id_tree'

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>