Current unclean leader election docs state:
In the future, we would like to make this configurable to better support use cases where downtime is preferable to inconsistency.
Since 0.8.2.0, unclean leader election strategy (whether to allow it or not) is already configurable via unclean.leader.election.enable broker config property.
That sentence is in both https://svn.apache.org/repos/asf/kafka/site/083/design.html and https://svn.apache.org/repos/asf/kafka/site/082/design.html near the end of "Unclean leader election: What if they all die?" section. Next section, "Availability and Durability Guarantees", mentions ability to disable unclean leader election, so likely just this one reference needs to be updated.