Clarify incident management issue templates

What does this MR do?

I wasn't sure precisely what the setting for a default template on incidents auto-created from alerts did: did it replace the default content (with templating somehow)?

From reading the code, it appends the template to the default content:

1. https://gitlab.com/gitlab-org/gitlab/-/blob/1e555be7369ee83c1e1ec779af5357a5994ab563/app/services/alert_management/create_alert_issue_service.rb#L45
2. https://gitlab.com/gitlab-org/gitlab/-/blob/1e555be7369ee83c1e1ec779af5357a5994ab563/app/presenters/alert_management/alert_presenter.rb#L26

Author's checklist

• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.