[Docs Update] GitLab.com Automatically Remove Dormant Members Edge Cases

See discussion: !157930 (comment 2250997208)

See how we define dormant members: #460840 (closed)

Problem

The following scenario can happen with this feature, which results in false positive identification of dormant members. We should document this, so customers are aware:

these records are being backfilled. So, it doesn't represent the correct date when the user was added as member.

1. So, user may be added as member 1.year.ago , and was active 60.days.ago, but doesn't have a record on user_seat_assignments table.
2. The backfill job runs and creates a new record for the user, with created_at: Time.now, say today.
3. 8.days.from_now , this user record will be treated as dormant and removed by this worker
4. But, it should be after ~30.days when it hits the 90.days of dormancy.

Disclaimers

The false positive for dormancy is more for new members who haven't been active in last 7.days criteria.

The 90.days or 120.days not active should not remove false positive, as we rely on last_activity_on timestamp explicitly.

Proposal

We inform customers that when enabling the feature, any users not active since the backfill was complete will be considered dormant, and it'll fall back to the dormant period (90d) after they have been active. We can provide the exact date for the backfill completion.

See draft text for docs here: #508941 (comment 2262825738)