dm-log-writes records the sequence of completion, not submission, thus
for the following sequence (W=write, C=complete):

  Wa,Wb,Wc,Cc,Ca,FLUSH,FUAd,Cb,CFLUSH,CFUAd

The logged results in log device should be:
  c,a,b,flush,fua

Fix the comment to give a better example.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>