[DOC] Fix validation comments for DS.Model#isValid

- Api docs included comments on failing client-side validation
- DS.model doesn't have client side validation
- Only code that sets errors and changes  is a result of an invalid response from server

Perhaps I just couldn't find it but I don't see any support for client-side validations in Ember Data but the documentation stated that a model can become invalid when client-side validations fail. It seemed confusing as I can't find any documentation or code on how ember data supports client side validations. See : http://emberjs.com/api/data/classes/DS.Model.html#property_isValid