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 
Maybe reword to:

"You can still -refresh manually even with this enabled to update the Realm in the middle of a function or at any time.