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 
If they have multiple schema versions they always have to call it on startup as there's no way to know if the realm is up to date (other than storing that information out-of-band), but I do agree that it's currently quite misleading.