block: document iostat changes for disk busy time accounting

Since commit 5b18b5a7 ("block: delete part_round_stats and switch to
less precise counting") io_ticks is approximated by adding one at each
start and end of requests if jiffies has changed.

This works perfectly for requests shorter than a jiffy. If requests runs
more than 2 jiffies some I/O time will not be accounted unless there are
other reuqests.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>