Something is "available" or not depending on the observer and point of view. Users should be able to position Sentinels accordingly to the kind of failure detection they want, and usually where the slave are is not an objective point of view (that is instead where clients are usually).
Position, number of sentinels, level of agreement. Change this three elements and you can create many kind of setups.
About operations: Sentinels are designed to monitor other sentinels automatically and are as simple as possible to operate.