API docs: use GraphQL types not JSONValue

Created by: sqs

I understand it is because "GraphQL has terrible support for recursive data structures: https://github.com/graphql/graphql-spec/issues/91". But the API would be much easier to use IMO if it were GraphQL-typed and not just JSONValue. The GraphQL query can just request a capped depth (the UI probably only supports a couple levels of nesting anyway). This is what the comment https://github.com/graphql/graphql-spec/issues/91#issuecomment-254895093 in reply to that issue also suggests.

(Just opening this up for discussion and to give 1 data point, it's obviously your call.)