onCompleted response field not raw as documented

The docs say the `response` arg in the onCompleted callback is the raw server response.
However, it's really just coming from a snapshot.
This is usually fine, unless my mutation payload includes a fragment, then it barfs because it does the standard colocating only-resolve-what-you-need thing & i end up with an object like `{_fragments: {Foo_foo: {}}}`.

This is related to #1995 as it's part of the bigger issue of writing maintainable mutations. If we can't use fragments in our mutations, that should probably be noted in the docs. (and hopefully there's a useful pattern to share so my requested mutation payload is an aggregate function of my query fields).