cfq-iosched: Add comments on update timing of weight

Explain that weight has to be updated on activation.
This complements previous fix e15693ef ("cfq-iosched: Fix wrong
children_weight calculation").

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <axboe@fb.com>