update docs for mocking

docs/source/apollo-server/mocking.md

@@ -15,3 +15,4 @@ For more information about how to use the `mockServer` function, see the [Medium
 </h3>
 
 `addMocksToSchema` is the function that `mockServer` uses under the hood. Given an instance of GraphQLSchema and a mock object, it modifies the schema in place to return mock data for any valid query that is sent to the server. If `mocks` is not passed, the defaults will be used for each of the scalar types. If `preserveResolvers` is set to `true`, existing resolve functions will not be overwritten to provide mock data. This can be used to mock some parts of the server and not others.
+`mockServer` will set the `context` to `{}` when invoking `graphql`, which means that the mock functions could use that context to store additional information for mocking, such as relationships between different types.