Fix incorrect documentation for model.isError

If the server returns a validation error, then isError is not set to true.

You can see the implementation below (which only calls recordWasError if there is no validation errors):
https://github.com/emberjs/data/blob/20adb1d5b0db0058b9fda35265c9916f2fe7c964/packages/ember-data/lib/system/store.js#L1781

This is further documented in the comment for recordWasError:
https://github.com/emberjs/data/blob/20adb1d5b0db0058b9fda35265c9916f2fe7c964/packages/ember-data/lib/system/store.js#L1011

Note: I linked the currently most recent commit in case the line numbers end up changing.