add note on setupTestServerContext

test/testHelpers/apolloTestServerHelpers.ts

@@ -17,6 +17,10 @@ function setUpTestServer() {
   });
 }
 
+/**
+ * Returns a `ServerContext` object which can be passed to requests
+ * for testing.
+ */
 export function setupTestServerContext() {
   const context: { [key: string] : any } = {};