Add information about what's in chains

[mhauru]

The Core Fuctionality page should mention under the heading Analysing MCMC chains that the chain also has the log probs and diagnostic information in it (even if this might not be the right place to go into full detail). Also, later under the Querying Probabilities from Model or Chain heading we should talk more about log probs being in the chain and when you do/do not need to call something like logjoint on the chain.