Document Pro threshold billing behavior

[morgankrey]

This updates the account docs to reflect changed 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

[morgankrey]

Superseded by #58320, which applies the same docs update to the current main docs paths.