[doc] hasRecordForId takes a `DS.Model`, not a `String`

Perhaps the real fix is to use modelFor here, but hasRecordForId is also used in `recordIsLoaded` which require a subclass of DS.Model.
To go further, perhaps all public methods should be consistent, I mean take a string parameters and call modelFor.
Merge pull request #1413 from sly7-7/patch-1

[doc] hasRecordForId takes a `DS.Model`, not a `String`