Document the pull_requests array's behavior in the checks event (github#15780)

Author: BenEmdon

data/reusables/webhooks/check_run_properties.md

@@ -6,5 +6,6 @@ Key | Type | Description
 `check_run[conclusion]`|`string` | The result of the completed check run. Can be one of `success`, `failure`, `neutral`, `cancelled`, `timed_out`,  {% if currentVersion == "free-pro-team@latest" or currentVersion ver_gt "[email protected]" %}`action_required` or `stale`{% else %}or `action_required`{% endif %}. This value will be `null` until the check run has `completed`.
 `check_run[name]`|`string` | The name of the check run.
 `check_run[check_suite][id]`|`integer` | The id of the check suite that this check run is part of.
+`check_run[check_suite][pull_requests]`|`array`| An array of pull requests that match this check suite. A pull request matches a check suite if they have the same `head_sha` and `head_branch`. When the check suite's `head_branch` is in a forked repository it will be `null` and the `pull_requests` array will be empty.
 `requested_action`|`object` | The action requested by the user.
 `requested_action[identifier]`|`string` | The integrator reference of the action requested by the user.