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 
The above is much better but should we discuss what "change" means? Is an empty write transaction a change? Is adding and removing an object in a transaction a change?