Document RLMRealm lifetimes and related issues

Should cover all of the points of confusion related to RLMRealm lifetimes that people have run in to.
This does seem a bit fragile. Wouldn't it make more sense to encourage users to always store the realm in their app delegate or somewhere similar, and use that throughout their app? The fact that you sometimes get the same realm and sometimes not seem fraught with opportunities for hard to find bugs. It would seem much safer if we could encourage a pattern where they always keep a realm active for thread that needs it.