Fix incident noteable_kind and update comment

What does this MR do and why?

The IncidentEvent class is missing the implementation of notealbe_kind which is the root cause for broken_master_label_nudger has not been invoked... (#1376 - closed).

This MR fixes the noteable_kind for incident events, and I have also updated the automation comment to clarify that if all of the necessary labels are added, they can ignore the comment. This is to handle the scenario where team members cannot choose a concrete flaky test label, or if a race condition was caused by team member labeling and closing the incident via quick action.

Closes broken_master_label_nudger has not been invoked... (#1376 - closed)

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review