This updates docs to reflect changes threshold billing behavior for
individual Zed Pro subscriptions.

- Scope threshold billing language explicitly to individual Zed Pro
subscriptions.
- Remove stale language saying each $10 threshold crossing resets the
threshold to $0.
- Document that threshold invoices start at $10, may rise in $10
increments up to $100, and are not automatically lowered during the same
subscription.
- Split spend-limit docs between Zed Pro and Zed Business, with Business
readers linked to organization billing.

Release Notes:

- N/A