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 
If no objects require alteration, then enumerating all objects which require alteration and then returning a new version consists of just returning a new version. If some do require alteration (e.g. if you added a string property with no default), then just returning a new version will not work. This could probably be worded better, but I didn't have any ideas that aren't inane.