docs: block/bfq: describe per-device weight

The functionality of setting per-device weight for BFQ was added
in v5.4 (commit 795fe54c

), but the documentation was never
updated.

While at it, improve formatting a bit.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Link: https://lore.kernel.org/r/20210614214109.207430-1-kolyshkin@gmail.com
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>