Improve documentation for Jira Connect OAuth feature rollout

We are currently holding off releasing the jira_connect_oauth feature flag because it would be a breaking change for self-managed users using the manual installation workaround.

Users will not be able to authorize unless an instance admin adjusts settings using the following steps:

1. On the top bar, select Main menu > Admin.
2. On the left sidebar, select Applications (/admin/applications).
3. Select New application.
4. In Redirect URI, enter https://gitlab.com/-/jira_connect/oauth_callbacks.
5. Ensure the Trusted and Confidential checkboxes are cleared.
6. In Scopes, select the api checkbox only.
7. Select Save application.
8. Copy the Application ID value.
9. On the left sidebar, select Settings > General (/admin/application_settings/general).
10. Expand the GitLab for Jira App section.
11. Paste the Application ID value into Jira Connect Application ID.
12. Select Save changes.

Those steps should be documented here