blktrace: update blkiomon doc

blktrace: update blkiomon doc

Fixup for 499398.

Description of problem:

blkiomon does not understand the output of blktrace when
working with logical volume device (it is quiet, while
working with physical device it prints IO statistics as
expected).

BUG (or design feature?):
/dev/dm-* and /dev/md* don't see BLK_TC_COMPLETE actions:

        /* we need an older D trace and a younger C trace */
        if (t_old->bit.action & BLK_TC_ACT(BLK_TC_ISSUE) &&
            t_young->bit.action & BLK_TC_ACT(BLK_TC_COMPLETE)) {
                /* matching D and C traces - update statistics */
                match++;
                blkiomon_account(&t_old->bit, &t_young->bit);
                blkiomon_free_trace(t_stored);
                return t;
        }

Possible solution:
Update documentation.

Signed-off-by: Edward Shishkin <edward@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>