Document gracefully handling of generic alert payloads

Follow-up of https://gitlab.com/gitlab-org/gitlab/-/merge_requests/53999

doc/operations/incident_management/integrations.md

@@ -90,7 +90,7 @@ parameters. All fields are optional. If the incoming alert does not contain a va
 | `service`                 | String          | The affected service. |
 | `monitoring_tool`         | String          |  The name of the associated monitoring tool. |
 | `hosts`                   | String or Array | One or more hosts, as to where this incident occurred. |
-| `severity`                | String          | The severity of the alert. Must be one of `critical`, `high`, `medium`, `low`, `info`, `unknown`. Default is `critical`. |
+| `severity`                | String          | The severity of the alert. Case-insensitive. Can be one of: `critical`, `high`, `medium`, `low`, `info`, `unknown`. Defaults to `critical` if missing or value is not in this list. |
 | `fingerprint`             | String or Array | The unique identifier of the alert. This can be used to group occurrences of the same alert. |
 | `gitlab_environment_name` | String          | The name of the associated GitLab [environment](../../ci/environments/index.md). Required to [display alerts on a dashboard](../../user/operations_dashboard/index.md#adding-a-project-to-the-dashboard). |