Add note that get_failover_log/2 is only used for testing

Change-Id: I432b62e3867c88a19a74daaffb7f73ae8e6da244
Reviewed-on: http://review.couchbase.org/33752
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>