Chapter on validation functions doesn't explain how to let deletes pass

[Bjoern]

I wrote a validation function but then found I could not delete documents anymore, because delete requests would not pass the validation.

I suspect there is a property _deleted of the newDoc parameter, which I saw elsewhere in an example. Not sure though - in the end I could not get it to work in the short time I had (nodeknockout...). But I think the solution should be mentioned in the chapter on validation functions.