Document RLMRealm lifetimes and related issues

Should cover all of the points of confusion related to RLMRealm lifetimes that people have run in to.
>  Is there a section with Realm lifecycles that we can link to?

There isn't. I started writing one in the class overview, but having the class overview be just a discussion of the RLMRealm lifecycle was bad, and writing a real overview would be a much bigger project.

> Might also be worth mentioning somewhere that objects obtained from a realm as well as notification tokens maintain a strong reference

That's noted a few sentences later.