[Support Feedback] Improve payment, invoice, and subscription docs (Billing)

[dmmulroy]

Summary

Improves billing documentation for high-volume support topics, based on an audit of active support macros.

Changes

• Failed payments: Document the 5-retry/5-day grace period and what happens when payment fails
• Payment methods: Clarify accepted payment types and card validation requirements
• Plan downgrades: Explain behavior when downgrading (feature removal timing, credit handling)
• Usage-based billing: Improve explanation of metered billing cycle and lag
• Invoices: Clarify how to access and interpret invoices
• Billing info updates: Document how to update payment and billing contact details
• Subscription transfers: Explain zone credit behavior and subscription portability
• Zero Trust free plan: Clarify billing expectations for the free ZT tier

Context

These changes are driven by recurring support cases. If you want to see the underlying support data (macro frequency, case volume by topic), reach out to @dmmulroy internally.

[github-actions]

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