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 
IMO duplicating it is friendlier to the user command-clicking the method and reading the header, but maintaining two copies of the description does sound pretty annoying. It looks like Apple's docs just have a reference to the first one and a description of the differences, so I guess I'll go with that.