PYCBC-254: Document lock as requiring TTL

Change-Id: I22d7e6064929485e59ae363d8117c8df184b4ff2
Reviewed-on: http://review.couchbase.org/40057
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>