dm flakey: Document "error_writes" feature

Commit ef548c55 ("dm flakey: introduce "error_writes" feature")
added the ability to dm flakey to error out writes in contrast to
silently dropping it with 'drop_writes'. Unfortunately this feature
is not currently documented and one has to be either familiar with the
source code of dm flakey or check out xfstests sources to know of
this parameter. So document it.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>