Improve the RLMRealm documentation a bit

Fix a pile of small issues and remove the claims that autorefresh makes refreshing and notifications happen "immediately" as they're actually only enqueued immediately, and go into a bit more detail about how things work.

@alazier @jpsim 
This could easily be misunderstood to mean that you HAVE to call this before accessing the Realm (even if you have no need for migrations). It should probably be made clear that it is perfectly ok to open Realms without doing the migration step first (as long as they are up-to-date with the model), but if you need to do migrations, it has to be done before the first opening of the Realm.