Fix seat usage alert threshold description

What does this MR do?

Clarify/fix "Seat usage alerts" threshold in the documentation

Related issues

(I did not open an issue for this small change)

Here we can find how the logic is defined https://gitlab.com/gitlab-org/gitlab/-/blob/54fdc621dfb084be7dcab2d005738a9d6cbeb9cb/ee/app/services/gitlab_subscriptions/reconciliations/calculate_seat_count_data_service.rb (introduced in !83492 (diffs))

For example, on an instance with 50 seats:

• The current wording "10% of seats have been used" can be interpreted as "I will be alerted when I reach 5 seats used", while the user will be alerted when 45 seats are used (so 10% left)
• The new wording "10% of seats in the subscription are remaining" should be clearer

Note that there may be an even clearer way to express it and I'm fine to use a clearer wording.

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