include optional exception when mentioning ScenarioResult

Helps with #1245

docs/support_files/api_reference.md

@@ -36,7 +36,7 @@ Defines a hook which is run after each scenario.
   * `tags`: string tag expression used to apply this hook to only specific scenarios. See [cucumber-tag-expressions](https://docs.cucumber.io/tag-expressions/) for more information
   * `timeout`: A hook-specific timeout, to override the default timeout.
 * `fn`: A function, defined as follows:
-  * The first argument will be an object of the form `{sourceLocation: {line, uri}, result: {duration, status}, pickle}`
+  * The first argument will be an object of the form `{sourceLocation: {line, uri}, result: {duration, status, exception?}, pickle}`
     * The pickle object comes from the [gherkin](https://github.com/cucumber/cucumber/tree/gherkin-v4.1.3/gherkin) library. See `testdata/good/*.pickles.ndjson` for examples of its structure.
   * When using the asynchronous callback interface, have one final argument for the callback function.