Clarify definition of response position

[benjie]

:: A response position is a uniquely identifiable position in the response
data produced during execution. It is either a direct entry in the {resultMap}
of a {ExecuteSelectionSet()}, or it is a position in a (potentially nested) List
value. Each response position is uniquely identifiable via a response path.

The intention of response position is that it includes things that were elided by error propagation; e.g. if you query { a { b } } and b is non-null and throws, the result will be {a: null} but the error will have path ["a", "b"] indicating the a.b response position even though that position does not actually exist in the response.

(Also: ExecuteSelectionSet() doesn't exist any more.)

This PR clarifies the definition of response position to include these omitted values.

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[martinbonnin]

👍