docs: Another small addition
model: Adding experimental support for caching policies with probability caching

This caching can be enabled, for example, for LRU policy with caching
probability 50%:

    ndnHelper.SetContentStore ("ns3::ndn::cs::Probability::Lru",
                               "MaxSize", "100",
                               "CacheProbability", "0.5");

Similar way, probability can be enabled for other existing caching policies.