Document the namespace compute minute and storage attributes

What does this MR do and why?

Four attributes of the namespace entity appear nowhere on this page: shared_runners_minutes_limit, extra_shared_runners_minutes_limit, additional_purchased_storage_size and additional_purchased_storage_ends_on. All four are exposed in ee/lib/ee/api/entities/namespace.rb under ->(namespace, opts) { ::Ability.allowed?(opts[:current_user], :update_subscription_limit, namespace) }.

They belong in the "Additional attributes might be returned for Group owners or on GitLab.com" example rather than in the main ones, because that block is already where the permission-gated attributes are shown, so that is where I put them, in the order the entity exposes them. I added one sentence under the block naming the permission, since "for Group owners or on GitLab.com" does not tell a reader which of the attributes in that block they will actually see.

The values are the entity's own documented examples.

Nothing else on the page needed changing: projects_count, root_repository_size, end_date, max_seats_used_changed_at, max_seats_used and seats_in_use are all already shown. There are no response attribute tables on this page, only request parameter tables.

I found this while adding the same four fields to the Go API client, in gitlab-org/api/client-go!3051, which is adding eight in total, the other four being ones this page already documents.

MR acceptance checklist

This change is documentation only. No endpoint, parameter or behaviour changes.

I parsed every JSON block on the page after editing. Four of the five parse unchanged. The fifth is the block I edited, which uses ... elisions and so is not valid JSON before or after this change; with the elisions removed it parses, and the object is the one shown above plus the four new keys.