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 
Apple uses NULL there since NSError** is a C thing rather than an Obj-C thing (i.e. you can't send messages to it).