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.

---
 doc/blkiomon.8 |    2 ++
 1 file changed, 2 insertions(+)

@@ -17,6 +17,8 @@ blkiomon is a block device I/O monitor. 
 request size and request latency statistics from blktrace data. It provides
 histograms as well as data that can be used to calculate min, max, average
 and variance. For this purpose, it consumes D and C traces read from stdin.
+Note, that this doesn't work for logical volumes, as high-level drivers
+don't see the completion of the events (C).
 
 There are options for binary output and human-readable output to files and
 stdout. Output to a message queue is supported as well.