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 is not correct. At a minimum, all you must do is a return a new version number. Al schema changes are made automatically and don't require you do do anything. In addition, you don't have to enumerate any objects, but you can if you need to update them.