Fixed errors in documentation

[rChaoz]

I would guess the code used to be for a ViewModel and was updated for an ItemViewModel, and the documentation hasn't been fully updated to reflect this; a few of these changes fix just that. Full change list:

• fixed model.item being referred to as model.person
• removed the additional save() function explanation that wasn't needed (there is no person field that needs to be changed to item)
• changed model.rebind() to model.rollback() as this is the code that is truly analogous to model.rebind { item = person2 }
• removed the passage that stated the PersonModel code is not included in the code (it is); as well as the passage in the PersonModel Implementation paragraph
• changed the PersonModel paragraph to explain the updated code (using Class::property instead of lambdas)
• more fixes in the code & documentation due to the ItemViewModel -> ViewModel change
• added explanations for the low-level ViewModel (since it is no longer used; the ItemViewModel is used and explained instead) and moved the paragraph explaining the differences here

[insolor]

• added explanations for the low-level ViewModel (since it is no longer used; the ItemViewModel is used and explained instead) and moved the paragraph explaining the differences here

Well, I was a bit confused because of mentioning ViewModel and using ItemViewModel instead in the guide. Now it makes much sense.