LU-1415 tests: Explain refresh_disk()

Following Brian's suggestion in http://review.whamcloud.com/2981, this
patch adds a brief comment on refresh_disk(), which could be a bit
tricky to understand at first sight.

Change-Id: I83d583b5149c7648be4c68e31b6ff31f57410d9f
Signed-off-by: Li Wei <liwei@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/3430
Tested-by: Hudson
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <tappro@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>