Update docs about minimal access being non-billable

Paulo Barrosrequested to merge
pb/330663/update-docs-minimal-access-non-billable into master

This is part of Make `minimal access users` or `users not assig... (#330663 - closed) and a follow-up to Make minimal access and users without membershi... (!210596 - merged).

What does this MR do and why?

Updates documentation following the merging of Make minimal access and users without membershi... (!210596 - merged) to reflect that minimal access users and users without memberships are now non-billable on all paid plans.

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