Document protection guarantees of new durability levels in admin guide

There will be 3 new durability levels in 6.5. The failure protection guarantees of these need to be documented.

The slide deck from Poonam Dhavale captures the details (slides 2,3,4 are the relevant ones from documentation point of view):

https://docs.google.com/presentation/d/1ijZ-7tYAdGypNh_aF0yC0KMMTqPXxAc-E5hUCCs82Ps/edit#slide=id.g4e8865a0ce_1_30

Action item on PM (Shivani Gupta) to condense the above into externally consumable format.