Docs Update: Clarify that licensee data is encrypted within license key in CL sync

What does this MR do and why?

Updates this section: https://docs.gitlab.com/ee/subscriptions/self_managed/#license-sync to clarify that licensee data is encrypted within the license key itself.

Review this section - are licensee name or email included in this CSV export: https://docs.gitlab.com/ee/subscriptions/self_managed/#export-your-license-usage

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