Document RLMRealm lifetimes and related issues

Should cover all of the points of confusion related to RLMRealm lifetimes that people have run in to.
Not caching RLMRealm instances would break all of the stuff that doesn't take a realm parameter and just operates on the default realm, and encouraging users to always get new RLMRealm instances rather than storing a reference makes multithreaded stuff a lot more natural (since storing a reference only really works for the main thread).