Clarify milestone expiration and closure

Why is this change being made?

I think the current revision is confusing and not properly reflecting with what we're doing now. See gitlab-org/quality/triage-ops#366 (comment 258921731)

Review: https://milestone-expire.about.gitlab-review.app/handbook/engineering/workflow/index.html#milestone-cleanup

The proposal will be implemented in:

• gitlab-org/quality/triage-ops!367 (merged)
• gitlab-org/quality/triage-ops!366 (merged)
• For now we can't easily triage against milestones: https://gitlab.com/gitlab-org/gitlab-triage/issues/39 but we can work on that.